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The traffic and behavioral effects of the I-35W Mississippi River
bridge collapse
Shanjiang Zhu
⇑
, David Levinson, Henry X. Liu, Kathleen Harder
University of Minnesota, Department of Civil Engineering, 500 Pillsbury Drive SE, Minneapolis, MN 55455, USA
article info
Article history:
Received 6 August 2008
Received in revised form 15 June 2010
Accepted 4 July 2010
Available online xxxx
Keywords:
I-35W bridge collapse
Travel behavior
Travel survey
abstract
On August 1, 2007, the collapse of the I-35W bridge over the Mississippi River in Minneap-
olis abruptly interrupted the usual route of about 140,000 daily vehicle trips, which sub-
stantially disturbed regular traffic flow patterns on the network. It took several weeks
for the network to re-equilibrate, during which period travelers continued to learn and
adjust their travel decisions. A good understanding of this process is crucial for traffic man-
agement and the design of mitigation schemes. Data from loop-detectors, bus ridership
statistics, and a survey are analyzed and compared, revealing the evolving traffic reactions
to the bridge collapse and how individual choices could help to explain such dynamics.
Findings on short-term traffic dynamics and behavioral reactions to this major network
disruption have important implications for traffic management in response to future
scenarios.
Ó2010 Elsevier Ltd. All rights reserved.
1. Introduction
On August 1, 2007, the collapse of the I-35W bridge over the Mississippi River in Minneapolis abruptly interrupted the
usual route of about 140,000 daily vehicle trips, which substantially disturbed regular traffic flow patterns on the network.
It took several weeks for the network to re-equilibrate, during which period travelers continued to learn and adjust their
travel decisions. This process is not well understood due to the limited number of major network disruptions. Moreover, traf-
fic monitoring devices are usually not readily available surrounding the disruption site. Consequently, data collection efforts
are limited in the aftermath of unpredictable disruptions. This may partly explain why most studies of major network dis-
ruptions look only at one snapshot in time, usually a relatively long time after the incident. Fewer studies look at what hap-
pens immediately after a network disruption.
This natural experiment of prolonged, large-scale network disruption provides a unique opportunity to investigate
how travelers react to such incidents over a short and medium time period. Since the re-equilibration process occurred
within a short time frame, this process, for the most part, excludes the effects of long-term decisions such as work and
residential location. After the bridge collapsed, a survey was conducted that targeted travelers who were likely to be
affected by this incident. Respondents reported their commute travel pattern over time. Traffic data from loop-detectors
and bus ridership during the same time periods were also collected. We compared the information from different
sources to reveal the evolving traffic reactions to the bridge collapse and how individual choices could help to explain
such dynamics.
0965-8564/$ - see front matter Ó2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tra.2010.07.001
⇑
Corresponding author. Tel.: +1 612 626 0024; fax: +1 612 626 7750.
E-mail addresses: zhuxx120@umn.edu (S. Zhu), dlevinson@umn.edu (D. Levinson), henryliu@umn.edu (H.X. Liu), harde015@umn.edu (K. Harder).
URL: http://nexus.umn.edu (D. Levinson).
Transportation Research Part A xxx (2010) xxx–xxx
Contents lists available at ScienceDirect
Transportation Research Part A
journal homepage: www.elsevier.com/locate/tra
Please cite this article in press as: Zhu, S., et al. The traffic and behavioral effects of the I-35W Mississippi River bridge collapse. Transport.
Res. Part A (2010), doi:10.1016/j.tra.2010.07.001
This paper investigates aggregate traffic patterns and underlying travelers preference in response to a major network dis-
ruption based on the data collected immediately after the memorable collapse of the I-35W bridge. After a general review of
existing research in this field, data from three different sources are presented in detail. Conclusions will be made after a dis-
cussion of findings.
2. Literature review
Network disruptions caused by infrastructure failure are unusual but not unknown. In recent years, we have seen the col-
lapse of the I-80 San Francisco-Oakland Bay Bridge and I-880 Cypress Street Viaduct due to the Loma Prieta Earthquake, the
Hatchie River Bridge in Tennessee, and the I-40 bridge at Webbers Falls, Oklahoma, among others. Networks may be dis-
rupted by other disasters (such as 9/11 or Hurricane Katrina), and more commonly, construction and maintenance work.
The impacts of such incidents may vary in both the geographic (i.e. a strike by an earthquake could partially damage links
over a large area, while a bridge collapse could cut a crucial link of the network completely) and time dimensions (i.e. road
maintenance could finish in days, while the I-35W bridge required more than one year to rebuild). Depending on the level of
disturbance, travelers have to explore the network and adjust their travel pattern accordingly. A good understanding of this
process is crucial for traffic management and the design of mitigation schemes. Moreover, findings of behavioral effects
could have significant implications for travel demand modeling, especially day-to-day travel demand.
However, empirical studies of traffic and behavioral effects after major network disruptions caused by these incidents are
limited. Researchers are more likely to focus on the supply side and address the network reliability issue. For example, Chang
and Nojima (2001) investigate the post-disaster transportation system performance after the 1989 Loma Prieta, 1994 North-
ridge, and 1995 Kobe earthquakes, using measures based on length of network open, total and areal accessibility. Other
researchers adopt a normative paradigm and investigate the optimal traffic regulation strategy after a major disaster (e.g.,
Sumalee and Kurauchi, 2006). Travel demand is simplified in these models and travelers are assumed to follow either a User
Equilibrium (UE) assumption or instructions from transportation agencies.
The UE assumption cannot capture the day-to-day traffic evolution after a major network disruption. Clegg (2007) shows
that a capacity reduction due to road construction generated an initial ‘‘over-reaction” effect followed by a ‘‘settling down”
effect, using license plate match data from the city of York, England. Oscillation of overall traffic and individual route choice
are reported. However, they also report that plate-matching is error-prone and more research is required to generate con-
vincing results. Cairnes and Goodwin (2002) investigate 70 case studies of road capacity reduction and conclude that
although people changed mode, consolidated trips for different purposes, and visited alternative destinations in response
to network degradation, ‘‘changing route and changing journey time seem to be the most universal”. They also indicate that
travelers are very adaptive and ‘‘wide-spread, long-term” disruption is rare. They argue travel behavior was conditioned on
new experience instead of past history. Their study, however, is more qualitative rather than quantitative.
In contrast, Hunt et al. (2002) evaluate travelers’ responses to a 14 month long closure of the Center Street Bridge in the
city of Calgary, Alberta, Canada, based on both traffic counts and results from a telephone survey. A minor drop (4.4%) in total
daily trips was reported and 39% of respondents left home early. Their traffic count data include only observations from two
days, in May 1999 and May 2000, respectively. The limited data prevent them from drawing statistically significant conclu-
sions. Moreover, the economic and social background of drivers may change significantly over a year, preventing them from
establishing any convincing causal effects. Nevertheless, their findings were echoed by Dahlgren (2002) who studied travel
behavior changes after the reopening of I-880 in Oakland, California ten years after it was destroyed by the Loma Prieta
earthquake in 1989. This research also found the popularity of route and departure time changes, and a minor but detectable
reduction in travel demand due to mode changes and cancellation of trips.
Giuliano and Golob (1998) and Wesemann et al. (1996) provide a comprehensive study of traffic and behavioral responses
after the 1994 Northridge Earthquake in metropolitan Los Angeles, California. However, earthquakes strike the entire net-
work of the affected region, generating greater consequences than single-point disruptions such as bridge collapses. The
day-to-day evolution in traffic pattern after bridge collapse is also different from evacuation scenarios such as those during
the Hurricane Katrina and Rita (Wolshon et al., 2006), which aim at moving affected population to a limited number of des-
tinations as soon as possible.
Thus, although significant efforts have been made to explore how travelers immediately respond to network disruptions,
a comprehensive study focusing on how overall traffic and individual travel patterns vary over time has not yet been re-
ported. Therefore, this research explores the traffic and behavioral effects over time after a major network disruption, using
data from both traffic detectors and a survey. Findings from this study could inform the day-to-day modeling of travel
demand.
3. Evolution in aggregate travel demand
After the I-35W bridge collapse, many travelers found alternative routes for their trips, both because of road or ramp clos-
ings and because of deteriorating traffic conditions on their normal route due to traffic reallocation across the network. As
evidence of the traffic reallocation process, we note the extreme hike in the number of vehicle trips on Minnesota Highway
280 and the I-94 bridge crossing the Mississippi River. This was the primary alternative route detour recommended by
2S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx
Please cite this article in press as: Zhu, S., et al. The traffic and behavioral effects of the I-35W Mississippi River bridge collapse. Transport.
Res. Part A (2010), doi:10.1016/j.tra.2010.07.001
Minnesota Department of Transportation (MnDOT). The average daily traffic on both directions of Minnesota Highway 280
increased from 25,000 trips in August to about 64,000 in October, while trips crossing the I-94 Mississippi River bridge in-
creased 26.36% during the same time period (see statistics in Table 3).
After the bridge collapse, a series of projects were implemented to add capacity and reduce bottlenecks. For example, an
additional lane was added in each direction of I-94 between I-35W and Mn 280 by reducing or eliminating shoulders and
restriping the road to provide narrower lanes. Mn 280, which had been a high-speed mostly limited access route with
two traffic signals, was converted to a freeway by closing several access points and reconstructing several interchange on
and off-ramps.
Most of these projects were finished by the middle of August 2007, while travelers were still exploring the network and
adjusting their travel pattern. By October, traffic flow became relatively stable. Wardrop’s first principle (Wardrop, 1952)
states that in equilibrium, ‘‘the journey times in all routes actually used are equal and less than those which would be expe-
rienced by a single vehicle on any unused route”. While this ideal equilibrium is never reached, since total demand varies
from day-to-day and drivers have imperfect information, it is approached. Duration of this re-equilibration process may ex-
tend from several days (Clegg, 2007) to one year (Hunt et al., 2002) depending on context, and in models has usually been
assumed without solid justification. Actually, it is very difficult to verify such equilibrium from the path travel time of each
origin–destination pair because of the heterogeneity of route choice criteria among the population, perception error, sub-
optimal behavior, and ever-changing network conditions. Instead, in this paper, traffic flow is used as a surrogate indicator,
a common practice in evaluating traffic assignment results. The Root Mean Square Error (RMSE), which measures the level of
deviation over time, is employed to measure the overall change of traffic. Traffic counts of 24 freeway stations, including
major freeway sections within the affected zone defined by the Minnesota Department of Transportation and all freeway
sections crossing the Mississippi River (locations are indicated in Fig. 1), were used to measure how significantly traffic var-
ied over time. Given traffic flow of two time periods, { f
t,1
,f
t,2
,...,f
t,n
} and {f
t+1,1
,f
t+1,2
,...,f
t+1,n
}, RMSE is defined as
RMSEðtþ1Þ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
n
i¼1
f
tþ1;i
f
t;i
ðÞ
2
N1
s
f
tþ1
Fig. 1. Traffic stations used for capturing the equilibration process.
S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx 3
Please cite this article in press as: Zhu, S., et al. The traffic and behavioral effects of the I-35W Mississippi River bridge collapse. Transport.
Res. Part A (2010), doi:10.1016/j.tra.2010.07.001
where {f
t,1
,f
t,2
,...,f
t,n
} and {f
t+1,1
,f
t+1,2
,...,f
t+1,n
} are average daily traffic counts within a week (Monday to Friday) at station i
during time period tand t+ 1, respectively. It has been found that trips such as grocery shopping, visiting friends are often
repeated on a weekly cycle (Clegg, 2007). Therefore, weekly average traffic counts are adopted here to avoid volatility caused
by within-week effects.
RMSEs over time from both 2006 and 2007 are summarized in Fig. 2. The exaggerated increase of RMSE in August 2007
suggests that traffic patterns varied significantly immediately after the bridge collapse. Then value of RMSE decreased over
the next few weeks, suggesting the network was approaching a new equilibrium. The RMSE fell within the range of normal
month-to-month variation in November, implying that a new traffic pattern was established by the end of October. In order
to evaluate the convergence with robust statistics, a t-test was conducted on the normalized weekly (Monday to Friday)
average traffic. The difference introduced by locations is ruled out by normalized weekly average traffic with two-year aver-
age during the study period (June to November), while the monthly and seasonal variations are controlled by comparing
2007 and 2006 data. If 0.1 is used as the critical value, the difference in normalized weekly traffic counts in 2007 are not
statistically significant compared to that of 2006 after the second week of September, which is consistent with conclusion
about the length of this re-equlibration process drawn from the RMSE measure.
Although the traffic pattern on the freeway system near I-35W bridge exhibited a detectable transient state, the overall
traffic demand did not change much after the bridge collapsed. Fig. 3 summarizes the on-ramp daily traffic counts from the
same time period during 2006 and 2007. We only show on-ramp data here because on-ramp detectors are better maintained
compared to off-ramp detectors due to their role in ramp meter control. Systematic bias could be observed between 2006
and 2007, due to absence of some detectors. However, it still indicates how overall travel demand evolved during the period
after the bridge collapse. A regression is built to test if the overall travel demand across the Twin Cities area has changed due
to the bridge collapse, with other confounding factors controlled. The model is as follows:
y
on
r
amp;t
¼b
0
þb
1
I
y
þb
2
I
I35
þb
3
I
Mon
þþb
8
I
Sat
þb
9
I
Aug
þb
10
I
sep
þ
e
0
0.05
0.1
0.15
0.2
0.25
JunWeek2
JunWeek4
JulWeek2
JulWeek4
AugWeek2
AugWeek4
SepWeek2
SepWeek4
OctWeek2
OctWeek4
NovWeek2
NovWeek4
RMSE
2006
2007
July 4th Bridge
Collapse
State Fair
Labor Day
Start of School
Thanksgiving
Fig. 2. Weekly changes of traffic counts measured by the Root Mean Square Error, traffic volatility returned to normal ‘‘equilibrium” levels by mid-October.
1500000
2000000
2500000
3000000
3500000
7/1/07
7/8/07
7/15/07
7/22/07
7/29/07
8/5/07
8/12/07
8/19/07
8/26/07
9/2/07
9/9/07
9/16/07
9/23/07
Daily trips
OnRamp_2007
OnRamp_2006
Bridge Collapse
Fig. 3. Estimated daily trips on the freeway system measured by on and off-ramp counts, trips in 2007 generally exceed trips in 2006, except for the week
after the Bridge collapse.
4S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx
Please cite this article in press as: Zhu, S., et al. The traffic and behavioral effects of the I-35W Mississippi River bridge collapse. Transport.
Res. Part A (2010), doi:10.1016/j.tra.2010.07.001
where y
t
is the demand captured by on-ramp loop-detectors, which is a function of dummy variables for the year (I
y
), day of
the week (I
Mon
to I
Sat
), month (I
Aug
and I
Sep
) and a dummy variable indicating the time period after the bridge collapse (I
I35
).
The results are summarized in Table 1. Although the negative sign for the bridge collapse dummy variable is consistent with
the hypothesis that the collapse reduced demand, it is not statistically significant. The month did not play a significant role
either. All other independent variables are statistically significant, showing important yearly and weekly variance in travel
demand. This is consistent with results obtained by regressing total daily traffic counts at all freeway main-lane stations on
the same set of independent variables (summarized in Table 2). The total freeway traffic increase after the bridge collapse
may be due to detour travel, it is, however, insignificant. Day of the week is most important factor, and the R
2
is 0.90, indi-
cating that the model captures most day-to-day variation in freeway traffic. The insignificance of the bridge collapse on total
travel demand could be explained by the size (most trips occurred outside of the impacted area defined by MnDOT) and the
redundancy (people could easily find detour routes and other destinations for their needs) of the network. This robustness of
total travel demand is consistent with previous research (Hunt et al., 2002) and survey result, which will be discussed later.
Total travel demand alone cannot capture the complexity in travel demand, especially losses people suffered when main-
taining similar demand level on a degraded network. On a saturated network (which is common in large metropolitan areas,
including the Twin Cities), small increases in traffic could significantly increase the level of delay. Therefore, travelers have to
either wait longer in the queue, or explore the redundant capacity in the off-peak period by leaving earlier or later. And this
shift in demand could result in new congestion in previous ‘‘off-peak period” as the ‘‘peak period” spreads. Fig. 4 summarizes
the total freeway Vehicle Kilometers of Travel (VKT) and Vehicle Hours of Travel (VHT) on the third Wednesday of each
month during 2006 and 2007. Although VKT remains flat after the bridge collapse, VHT increases. The average speed across
the network (the ratio between VKT and VHT) decreased significantly after the bridge collapse, implying a higher congestion
level on the network. The largest drop in average speed was observed on August 2nd, 2007, the day after the bridge collapse,
before any mitigation projects were implemented and before travelers could fully adapt to the degraded network.
If we focus on the network near the I-35W bridge, travel patterns exhibited more significant changes. Table 3 summarizes
daily trips on all bridges crossing the Mississippi River in the region (positions of these bridges are indicated in Fig. 1). The
arterial traffic counts after the bridge collapse were collected by the City of Minneapolis on two selected days of late Sep-
tember and early October. The data before the bridge collapse were AADT estimated every two years by the City of Minne-
apolis and MnDOT. The freeway data are from the loop-detectors on two days which are close to when the observations on
Table 1
Regression on daily travel demand measured by all on-ramp
loop-detectors.
Coefficient tStat
Intercept 2,116,765 27.41
***
Year = 2007 156,044 1.96
**
Collapse 57,568 0.59
August 36,450 0.49
September 23,943 0.32
Monday 300,685 3.58
***
Tuesday 832,817 9.92
***
Wednesday 992,530 11.54
***
Thursday 991,477 11.79
***
Friday 1,022,360 12.16
***
Saturday 607,372 7.23
***
**
Statistically significant at 5% level.
***
Statistically significant at 1% level.
Table 2
Regression on daily demand captured by all main-lane traffic
stations.
Coefficient tStat
Intercept 31,469,651 65.28
***
Year = 2007 274,628 0.56
Collapse 214,431 0.35
August 94,374 0.21
September 1,499,443 3.25
***
Monday 3,834,131 7.37
***
Tuesday 6,952,930 13.23
***
Wednesday 9,376,140 17.64
***
Thursday 10,032,068 18.89
***
Friday 10,252,360 19.50
***
Saturday 11,159,069 21.25
***
***
Statistically significant at 1% level.
S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx 5
Please cite this article in press as: Zhu, S., et al. The traffic and behavioral effects of the I-35W Mississippi River bridge collapse. Transport.
Res. Part A (2010), doi:10.1016/j.tra.2010.07.001
arterials were made. About one-third of the traffic switched to arterials to cross the river, while one third were diverted to
freeway alternatives, primarily the I-94 bridge. However, one-third of previous trips using the I-35W bridge disappeared,
which may be due to changes in destination and trip consolidation, and to a lesser extent changes in mode. These findings
are consistent with previous research (Goodwin, 1977; Giuliano and Golob, 1998) and survey results to be discussed.
4. Impacts for mode choice
Local media and practitioners expected immediate after the bridge collapse that the extra congestion following bridge
collapse would persuade a significant amount of travelers to switch to public transit. Conclusions about the impacts on tran-
sit ridership due to network disruptions could have important implications for traffic management. In the literature, travel-
ers show great resistance to changing mode (Hunt et al., 2002; Giuliano and Golob, 1998). As part of the mitigation plan in
response to the bridge collapse, Metro Transit, the primary public transit service provider at the Twin Cities, enhanced their
service by adding new buses and expanding capacity in park-and-ride lots. The monthly bus ridership was collected at the
fare box and documented by Metro Transit. Fig. 4 summarizes the monthly total trips during 2006 and 2007, on all routes
and on those routes crossing the Mississippi River, respectively. In order to detect the effects of the bridge collapse, a regres-
sion is built as follows:
y
t
¼b
0
þb
1
D
t
þb
2
I
y
þb
3
VKT
t
þbI
I35
þ
e
where y
t
is the monthly ridership, which is a function of the number of work days of the month (D
t
), dummy variables for the
year (I
y
), total Vehicle Kilometers Travel in the month (VKT
t
) and a dummy variable indicating the time period after the
bridge collapse (I
I35
). Table 4 summarizes the results and an R
2
value of 0.57 was obtained. The number of work days is
statistically significant and positively correlated with the monthly ridership. This is consistent with the fact that 48.3% of
80
85
90
95
100
105
110
0
50000
100000
150000
200000
250000
300000
350000
400000
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Speed(kilometer/hour)
VHT (veh*h)
VKT(100veh*km)
VHT 2007
VHT 2006
VKT/100 2007
VKT/100 2006
VHT (Aug 2nd 2007)
VKT (Aug 2nd 2007)
Speed 2007
Speed 2006
Speed (Aug 2nd 2007)
Fig. 4. Freeway Vehicle Kilometers of Travel (VKT) and Vehicle Hours of Travel (VHT) and average speed, estimated on the third Wednesday of each month.
Total travel (both VKT and VHT) was generally higher in 2007 than 2006, speeds varied, though August 2nd was considerably slower as many drivers were
on unfamiliar route in a dis-equilibrium condition.
Table 3
Traffic count on bridges crossing Mississippi River.
Bridge Before After Increase Percentage
Camdon St 8748 N/A
Broad St 20,931 N/A
Lowry St 16,296 N/A
Plymouth St 11,226 13,842 2616 23.30
Hennepin Ave 33,559 45,936 12,377 36.88
3rd Ave 31,303 39,423 8120 25.94
10th St 27,356 33,937 6581 24.06
Washington St 20,713 31,941 11,228 54.21
Franklin Ave 9800 13,061 3261 33.28
Ford Bridge 18,354 19,426 1072 5.84
I-694 160,426 168,645 8219 5.12
I-94 155,771 196,829 41,058 26.36
I-35E 87,251 88,032 781 0.90
TH-55 28,826 27,534 1292 4.48
I-35W 140,000 0 140,000 100.00
Arterial total 152,311 197,566 45,255 29.71
Freeway total 572,274 481,040 91,234 15.94
Total 724,585 678,606 45,979 6.35
6S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx
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Res. Part A (2010), doi:10.1016/j.tra.2010.07.001
total public transit trips are commute trips (home-based work, home-based work-related, home-based school and non-
home-based work) in the Twin Cities (seven county area) according to the 2000 Travel Behavior Inventory (TBI) conducted
by the Metropolitan Council. The impact of the bridge collapse is statistically significant, leading to a 6.6% increase in
monthly ridership according to the regression model. The year-to-year effect is positive, but not statistically significant.
The freeway VKT did not play a significant role either, suggesting different patterns exist for transit usage and freeway traffic.
Since except for expresses, most buses exclusively use arterial roads, transit is more of a substitute for arterial travel than
freeway travel. Similar results are obtained by regressing the total number of trips on all crossing-river routes, although
the magnitude of impacts due to the bridge collapse is larger (7.4%) according to the coefficients. However, bus riders on
these routes do not necessarily cross the river. Moreover, public transit trips represent only 2.3% of total trips in the seven
county area according to the 2000 TBI. Therefore, the effects of such increases in transit ridership are limited and probably
undetectable from traffic counts. The limited increase in transit ridership suggests that it is impractical to expect significant
changes in mode share under future scenarios.
5. Behavioral reactions
Travel demand emerges from individual decisions. In order to better understand traffic evolution and the equilibration
process after the bridge collapse, it is crucial to explore individual travel choices in the wake of major transportation network
disruption. A mail-in survey was conducted by the University of Minnesota, during September 2007. A copy of the question-
naire is attached in Appendix A, which includes four parts: questions about demographic information, self-evaluation of the
impacts on general travel patterns, travel choices regarding morning commute, and four maps on which respondents were
asked to draw their commute routes. In order to keep the survey salient, demographic questions include only gender and
family size, two factors which are likely to influence travel patterns. However, the residential and work locations, which
may link survey data with other data resources such as metropolitan planning models and thus inform route choice deci-
sions, were explicitly asked. Respondents were asked to assess whether they were affected by the bridge collapse in general,
and how they reacted (e.g did they cancel trips, avoid certain destinations, change departure time, mode, and/or route, en-
gage in more telecommuting, etc.). More specifically, questions were asked about their morning commute pattern, including
the departure time, the arrival time, travel mode and route choice during four phases: before the bridge collapse, the morn-
ing after the bridge collapse, the following weeks, and current status (at the time of the survey). Answers to these questions
illustrate how travelers adapted to network changes and inform the analysis about the underlying criteria for making these
decisions. Moreover, respondents were asked to identify their motivations for adjusting travel patterns and the resources
which assisted in those decision-making processes.
Changing route is one of the most easily adopted strategies to avoid unfavorable traffic conditions (as shown in the survey
results). However, it is very difficult for respondents to clearly and completely describe their commute routes in limited
words. Consequently it is challenging for researchers to correctly understand information conveyed through those words
and replicate the routes in analysis. Therefore, maps were adopted in this survey for drivers to communicate route informa-
tion. Four identical maps of the surface road network in the Twin Cities were included in the survey and respondents were
asked to draw their commute route during the four time periods previously defined. The maps included freeways, arterials,
and major streets, and were detailed enough to show major route changes. Respondents were asked to draw all routes they
frequently used during the four time periods. These routes, together with the reported residential and work locations, were
then digitized and compared in a geographic information software package (Arc/GIS). Meanwhile, traffic data during these
time periods, including both traffic counts and point speed, were collected from the loop-detector system of MnDOT, which
provides a good estimate of traffic conditions experienced by respondents in their day-to-day commute.
It is crucial to conduct a pretest in survey research to rule out ambiguity and confusions in survey questions and the de-
sign of questionnaires (Hunt et al., 1982). In order to develop a quality survey within this limited time frame, many of the
questions were adopted, with necessary revisions, from a previous survey study conducted by Levinson et al. (2006). We at-
tempted an ad-hoc pretest with ten students from the Department of Civil Engineering (not involved in this project). They
filled out the questionnaire. Researchers then probed these individuals for potential problems for the design of the question-
naire. Although no sign of uneasiness, confusion, and resistance were reported from our ad-hoc pretest, a more systematic
pretest would be advised in future research.
Table 4
Regression on monthly transit ridership.
Coefficient tStat
Intercept 1,759,581 1.85
*
WorkDays 122,085 2.44
**
2007/2006 27,159 0.22
VKT (million) 844.38 0.92
After collaspe 349,697 2.50
**
*
Statistically significant at 10% level.
**
Statistically significant at 5% level.
S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx 7
Please cite this article in press as: Zhu, S., et al. The traffic and behavioral effects of the I-35W Mississippi River bridge collapse. Transport.
Res. Part A (2010), doi:10.1016/j.tra.2010.07.001
The survey was distributed in both the downtown area of the City of Minneapolis and the nearby Minneapolis campus of
the University of Minnesota, Fig. 1 shows the relative locations (indicated by two red rectangles) of these two areas near the
I-35W bridge (the yellow dot) and thus significantly affected by the bridge collapse. According to the 2005 Longitudinal
Employment and Household Dynamics (LEHD) data, these two communities concentrated about 41.3% of total employment
(413,939) within the impacted zone defined by MnDOT after the bridge collapse, which is bounded by Minnesota Highways
100, 62, and 51 and Interstate Highway 694 with a radius of 5 miles (see Fig. 1). Constrained by the amount of time and
funding efforts available immediately after the bridge collapse, we concentrated resources in these two communities. Survey
questionnaires were randomly handed out on streets, at bus stops, and at the exits of structured parking ramps during the
workday afternoon of the first two weeks in September, 2007. A total of 1000 survey forms were handed out, and responses
came in through September and October. In all, 141 usable responses were received. Data were digitized and matched with
traffic data from MnDOT for further analysis.
5.1. Characteristics of respondents
Table 5 summarizes the basic characteristics of all respondents, while the residential and work locations are illustrated in
Fig. 5. Among the 141 respondents, 61.7% were female, which is significantly higher than the national average value (49.8%
according to 2000 Census). The average household size is 2.80, which is slightly higher than the average value in Minnesota,
2.52, according to 2000 Census. Considering the limited sample size, these biases should be acceptable. More importantly,
the geographic distribution of our respondents is well-dispersed across the Twin Cities area as shown in Fig. 5, which sug-
gests that they are representative of the impacts on travel patterns of the bridge collapse for individuals who work in areas
near the bridge.
The respondents covered the full spectrum of travel modes. Single drivers (56.03%) represent the largest share of travel
mode. Considering the strict parking restrictions and high level of service of public transit in the two survey areas, it is not
surprising to see a large share of bus riders (26.95%). According to the 2000 Census, 25% of daily commuting trips to down-
town Minneapolis are made through public transit, and according to the 2000/01 Travel Behavior Inventory conducted by
the Metropolitan Council, 44% of peak hour commuting trips are by transit (Levinson and Krizek, 2008), while transit carries
Table 5
General characteristics of survey respondents.
Counts Percentage
Gender Female 87 61.70
Male 48 34.04
N/A 6 4.26
Household size 1 17 12.06
2 50 35.46
3 22 15.60
4 30 21.28
5 10 7.09
P6 6 4.26
N/A 6 4.26
Average 2.8
No. of children (age 0–5) 0 105 74.47
1 16 11.35
2 6 4.26
P3 1 0.71
N/A 19 13.48
Average 0.19
No. of children (age 6–16) 0 89 63.12
1 15 10.64
2 15 10.64
P3 7 4.96
N/A 15 10.64
Average 0.49
Commute mode Drive alone 79 56.03
Carpool/vanpool 10 7.09
Bus 38 26.95
Bike 2 1.42
Walk 2 1.42
Park and Ride 6 4.26
N/A 4 2.84
Flexibility in work schedule Yes 77 54.6
No 56 39.7
N/A 8 5.7
8S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx
Please cite this article in press as: Zhu, S., et al. The traffic and behavioral effects of the I-35W Mississippi River bridge collapse. Transport.
Res. Part A (2010), doi:10.1016/j.tra.2010.07.001
24% of all trips to campus according to the University of Minnesota, Parking and Transportation Service. From the survey, bus
riders (including park-and-ride) represent 20.2% of respondents commuting to the University of Minnesota campus and 48%
of those commuting to downtown Minneapolis. The share of transit riders to downtown in our survey is significantly higher
than the daily average, but close to the peak hour value. The diurnal graph of departure and arrival time shows 84% of
respondents left home during the morning peak period (6:00 am–9:00 am). Therefore, our survey result is roughly consistent
with overall mode shares among commuters. Regarding the flexibility of work schedules, 54.6% of respondents responded
positively, while 39.7% stated that they had no flexibility in work schedule.
5.2. Changes in general commute patterns after bridge collapse
Table 6 illustrates the impact of the I-35W bridge collapse on general travel patterns and reactions among respondents.
Among 141 respondents, 55% claimed that they were affected by this incident. The most widely adopted adjustments in tra-
vel pattern is to change the departure time (75% among those affected) and to change route (73% among those affected).
About 61% of those respondents categorized as affected chose alternative destinations for their activities, while some chose
an alternative mode (6%) or telecommuting (9%). These adjustments in travel pattern are not mutually exclusive, thus trav-
elers could make multiple changes simultaneously. Observations from the survey are consistent with our assumptions that
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Table 6
Reactions to the bridge collapse by mode.
Total Affected (%) Cancel trips (%) Avoid dest. (%) Change DT (%) Mode (%) Route (%) Work at home (%)
Total 141 54.61 7.80 33.33 41.13 3.55 39.72 4.96
Affected 77 14.29 61.04 75.32 6.49 72.73 9.09
Single driver 79 65.82 8.86 39.24 51.90 3.80 50.63 7.59
Carpooler 10 60.00 10.00 20.00 30.00 10.00 60.00 0.00
Bus rider 38 42.11 2.63 28.95 28.95 0.00 15.79 0.00
Bicyclist 2 50.00 50.00 50.00 50.00 0.00 50.00 0.00
Walk 2 0.00
S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx 9
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Res. Part A (2010), doi:10.1016/j.tra.2010.07.001
the departure time and travel route are the easiest adjustments travelers can make in the wake of a major sudden network
disruption.
Individuals resist changes in travel mode because it involves major shifts in lifestyle and is constrained by other factors
such as service availability and car ownership. However, better coverage and higher frequency public transit encourages
people to ride buses. For example, two respondents indicated in comments that the improved express bus lines and dis-
counted annual bus pass helped to persuade them to ride the bus.
Although a high percentage of respondents reported they chose alternative destinations to satisfy their needs, this num-
ber is more likely to represent non-commute trips such as visiting friends or going shopping. Few respondents reacted by
increasing telecommuting (Work at home) and it is unlikely that people would make long-term adjustments such as work
or residential location changes in such a short time period.
Individual choices produce trends that are consistent with observations of aggregate traffic patterns. It is not surprising to
see the overall travel demand remains stable since very few people forgo trips or choose telecommuting. Furthermore, route
choice and departure time decisions only affect the distribution of trips. They do not influence aggregate travel demand. Nev-
ertheless, the reaction of choosing alternative destinations to satisfy their needs helps to explain the drop in the number of
crossing-river trips.
If we evaluate reactions by mode, 66% of single drivers and 60% of carpoolers claimed to be influenced by the bridge col-
lapse, the highest among all modes. It is not surprising that the percentage of bus riders who were affected was much lower
(42%), since very few routes used this bridge before it collapsed. Notably, buses could benefit from driving on hard shoulders
in the peak hours prior to the bridge collapse, but after the widening and restriping of I-94, the hard shoulders became reg-
ular travel lanes, simultaneously increasing overall speed but decreasing the speed advantage that buses possessed. Single
drivers adapted to the network disruption by changing routes (51%) and departure time (52%), and less frequently destina-
tions (39%). However, carpoolers were more likely to change routes (60%) while less likely to change departure time (30%) or
destinations (20%) according to the survey. Carpoolers may have less flexibility in departure time and destination in order to
achieve coordination. Constrained by the service provided, bus riders are unlikely to change route (16% according to the sur-
vey). Shift rates in destination and departure time are 29% for both. Due to limited number of observations, the trends in
bicyclist are not representative.
5.3. Shifts in morning commute
Table 7 summarizes the travel mode, the departure time, and the arrival time of morning commute over time illustrating
major changes in wake of the bridge collapse. As previously indicated, the mode share is very stable despite the disturbance
on the network and no significant changes were detected. In contrast, a significant share of respondents (27.7%) indicated
Table 7
Changes in commute travel pattern.
July Aug 2nd Weeks ater End of Sep.
Travel mode (%)
Drive alone 56.03 51.77 54.61 54.61
Carpool/vanpool 7.09 7.09 6.38 7.09
Bus 26.95 26.24 26.24 27.66
Bike 1.42 2.13 2.13 1.42
Walk 1.42 1.42 1.42 2.13
Park and Ride 4.26 4.26 4.26 4.96
N/A 2.84 7.09 4.96 2.13
Departure time
Depart later (%) 4.3 7.1 9.9
No change (%) 58.9 66.0 68.1
Depart earlier (%) 27.7 20.6 17.7
N/A 9.2 6.4 4.3
Average advances in depature (min) 16.5 13.4 19.7
Average delays in depature (min) 61.7 50.5 40.4
Arrival time
Arrive later (%) 21.3 20.6 22.0
No change (%) 60.3 65.2 63.1
Arrive earlier (%) 9.9 5.7 7.1
N/A 8.5 8.5 7.8
Average advances in arrival (min) 15.00 12.00 24.89
Average delays in arrival (min) 28.00 31.00 30.35
Commuting time
Average (min) 35.62 40.18 38.63 38.74
Changes compared 5.13 3.21 3.20
to the length in July (min)
10 S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx
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that they departed earlier than usual on the day after the bridge collapse, and the percentage dropped slowly thereafter
(17.7% by the end of September). Only 8 respondents (5.7%) cancelled their commute trips on August 2nd.
However, a few travelers (4.3% on August 2nd) chose to depart later than usual and that percentage increased slightly
since then. Considering the percentage of respondents with flexibility in work schedule (54.6%), many travelers may choose
to postpone their morning commute to avoid the congestion in peak hours. However, travelers still prefer early departure
compared to late departure, showing a preference for a standard work schedule.
Similarly, a relatively constant share of respondents (about 22.0%) arrived later than their usual schedule before the
bridge collapse, while fewer then 10% arrived earlier to work. Later departures increased over the two month period com-
pared to pre-collapse, while earlier departures (compared to pre-collapse) declined over this period.
The individual commuting time can be derived by comparing the self-reported departure time and arrival time. As indi-
cated in Table 7, the average commuting time to work before the bridge collapse was 35.62 min, which is well above the
average travel time to work for the City of Minneapolis (24 min for home-based work trips according to 2000 Metropolitan
Council TBI data).
On August 2nd, average commuting time jumped to 40.18 min, which resulted from the traffic restrictions imposed to
facilitate the rescue work and unfamiliarity with the new network conditions among commuters on the day following
the bridge collapse. The average travel time reduced to about 38 min on average and remained constant since then. The dis-
turbance caused by the bridge collapse thus cost each traveler in the survey population 3.2 min in the morning commute.
However, the experienced delay is not evenly distributed among population. Comparing the self-reporting commute time
before and after the bridge collapse (summarized in Fig. 6), most respondents (about 52.8%) did not feel significant delay
on August 2nd, while this number increased to 61.7% by the end of September. Some respondents (as many as 8.4% by
the end of September) were better off because of traffic reallocation (e.g. those who use I-35W but exited upstream of
the bridge, or those who benefitted from improvements to I-94 and Mn280).
5.4. Information and motivations
Motivations for adjusting travel patterns and information resources used are documented in Table 8. About half of the
respondents adapted themselves to the evolving network conditions to save travel time. The number of travelers who ad-
justed their travel behavior because of road or ramp closure peaked on August 2nd (23.4%), while this number dropped
as many roads were reopened or expanded in the following weeks. However, there are still about 13% of respondents
who changed their travel pattern for reasons not listed in this survey.
People are likely to make judgements based on what they see. The day-to-day travel experience about network conditions
was the primary source of information, used by two-thirds of respondents. One-third of respondents relied on radio for infor-
mation, and one-fourth on television. Television, more than radio, is likely to influence the before-trip decisions. Web pages
were used by about one-sixth of travelers.
Social contacts such as family members, neighbors, and co-workers were used by 13.5% of respondents as information
resources. This number increased slightly after the bridge collapse, possibly reflecting more positive search efforts. However,
this share returned to the original level before the bridge collapse by the end of September when a new equilibrium was
established and search efforts reduced.
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
-35
-25
-15
-5
5
15
25
35
45
55
65
75
85
Percentage
Delay (Minutes)
August 2nd
Following Weeks
End of September
Fig. 6. Histogram of survey reported changes in travel time (after – before), most drivers saw no change, some drivers saw an increase in travel time, and a
few reported a decrease in travel time.
S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx 11
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Surprisingly, only one of all the 141 respondents stated that calling 511 helped make travel decisions. Although cell-
phones are readily available both at home and on route, the 511 program is apparently not widely used in Minnesota.
5.5. Commute routes
In order to identify any changes in travel routes, respondents were asked to draw their commute routes during four time
periods: before the bridge collapse, August 2nd, the weeks following the bridge collapse, and late September when this sur-
vey was conducted. Although 141 replies were received, we were only able to identify routes choices from 60 respondents.
Most bus riders did not draw the commute routes on maps. This may be because bus riders generally focus more on the loca-
tion of boarding and alighting, while paying less or no attention to the route buses are actually traveling. Moreover, many
people who perceived little impact from the bridge collapse simply ignored the request to draw maps because they thought
this was irrelevant. This reason could also help to explain the low reporting rate among bus riders since they were much less
likely to change commute route.
Usually, only one route was reported for any given time period per respondent, indicating a reluctance to frequently
switch routes. However, 34 travelers reported more than one route, illustrating the exploration process. Some 60 routes were
identified for the period before August 1st and the final time period, although some of them may be identical, indicating trav-
elers switched back to their original routes. In the interim, 18 and 16 routes were reported for August 2nd and the weeks
after, respectively.
6. Conclusion
Despite the heavy losses in life and injury, as well as the psychological shock, the collapse of I-35W bridge did not disas-
trously disrupt the overall traffic of Twin Cities network as initially predicted by the mass media. Travelers exhibited great
flexibility in dealing with the changed traffic pattern, although some encountered more inconvenience than others. Both the
survey data and traffic counts suggested that total travel demand did not significantly reduce after the network collapse, pos-
sibly because of redundant capacity provided by alternatives: The capacity of the nearby I-94 Mississippi River bridge was
expanded by restriping the shoulders as an additional lane in each direction; arterial bridges sustained heavier traffic; non-
peak period capacity got better use, all of which helped to compensate for the capacity losses. However, the stability in total
demand did not suggest no losses for travelers. Instead, the total travel time is clearly longer on average for those commuting
to downtown or the nearby University of Minnesota. The spread of the peak period on the I-94 bridge is more strong evi-
dence. Moreover, the rerouting process and higher level of congestion could imply losses of opportunities, which is impor-
tant but subtle to detect. About one-third of all trips which had used the I-35W bridge before the tragedy no longer crossed
the river, implying losses of economic benefit associated with those trips.
Apparently, the bridge collapse did not generate a large increase in public transit ridership, which is consistent with
observations in previous research (Giuliano and Golob, 1998). The difficulty in promoting public transit could be due to tra-
vel inertia (Goodwin, 1977) and flexibility for family needs (e.g. pick up child at day care). However, both Dimitriou et al.
(2006) and Hensher and Brewer (2002) argued that major incidents provide good opportunities for travelers to experience
alternative modes, which if successful could promote their use over the long-term. Therefore, follow-up studies are needed
to investigate impacts of bridge collapse on transit ridership.
Evidence from both traffic observations and survey results clearly indicated that changing routes and changing journey
departure time are the most common responses to the bridge collapse. Travelers make these decisions either simultaneously
or in sequence. Future research efforts are needed to disentangle them.
Table 8
Reasons for changing travel pattern and information resources assisting decision-making (%).
July Aug 2nd Weeks after End of Sep.
Motivations
Save travel time 49.6 41.8 46.8 50.4
Road or ramp closure 12.1 23.4 19.9 17.7
Others 12.1 12.1 13.5 14.9
N/A 36.2 31.2 30.5 27.7
Information resources
Experience 64.5 54.6 61.7 64.5
Call 511 0.7 0.7 0.7 0.7
Website 17.7 22.7 21.3 20.6
TV 27.0 29.1 24.8 24.8
Radio 34.8 38.3 35.5 35.5
Social contacts 13.5 14.9 16.3 13.5
Co-workers 5.7 7.1 8.5 7.1
Neighbors 1.4 1.4 2.1 2.1
Family 7.8 7.8 7.1 6.4
Others 1.4 2.8 2.1 3.5
12 S. Zhu et al. / Transportation Research Part A xxx (2010) xxx–xxx
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Res. Part A (2010), doi:10.1016/j.tra.2010.07.001
