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Comparing General Mobility and Mobility by Car

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In the last years, the emergence of big data led scientists from diverse disciplines toward the study of the laws underlying human mobility. Although these recent discoveries have shed light on very interesting and fascinating aspects about people movements, they are generally focused on global and general mobility patterns. For this reason, they do not necessarily capture phenomena related to specific types of mobility, such as mobility by car, by public transportations means, by foot and so on. In this work, we aim to compare general human mobility with mobility expressed by a specific conveyance, trying to address the following question: What are the differences between general mobility and mobility by car? To answer this question, we present the results of an analysis performed on a big mobile phone dataset and on a GPS dataset storing information about car travels in Italy.
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Comparing general mobility and mobility by car
Luca Pappalardo∗‡, Filippo Simini, Salvatore Rinzivillo, Dino Pedreschiand Fosca Giannotti
KDD Lab, Department of Computer Science
University of Pisa, Italy
Email: {lpappalardo, pedre}@di.unipi.it
Institute of Physics, Budapest University of Technology and Economics
Budapest, Hungary
Email: f.simini@neu.edu
KDD Lab, ISTI-CNR
Pisa, Italy
Email: {rinzivillo, fosca.giannotti}@isti.cnr.it
Abstract—In the last years, the emergence of big data led
scientists from diverse disciplines toward the study of the laws
underlying human mobility. Although these recent discoveries
have shed light on very interesting and fascinating aspects about
people movements, they are generally focused on global and
general mobility patterns. For this reason, they do not necessarily
capture phenomena related to specific types of mobility, such as
mobility by car, by public transportations means, by foot and so
on. In this work, we aim to compare general human mobility with
mobility expressed by a specific conveyance, trying to address
the following question: What are the differences between general
mobility and mobility by car? To answer this question, we present
the results of an analysis performed on a big mobile phone dataset
and on a GPS dataset storing information about car travels in
Italy.
I. INTRODUCTION
In the last few years, the emergence of big data led scien-
tists from diverse disciplines toward the study of human mo-
bility, helping to discover and understand the hidden patterns
in the trajectories people follow during their daily life. Such
a social microscope showed that traditional mobility models
adapted from the observation of particles or animals (such
as Brownian motion and L´
evy-flights) [1], [2] and recently
from the observation of dollar bills [5], are not suitable to
describe people’s movements. Indeed, at a global scale humans
are characterized by a huge heterogeneity, since a power law
was observed in the distribution of the characteristic distance
traveled by users [3], [4]. Despite the observed heterogeneity in
people’s movements, through the observation of past mobility
history the whereabouts of most individuals can be predicted
with a very high accuracy, higher than 80% [6], [7].
These recent discoveries have undoubtedly shed light on
very interesting and fascinating aspects about human mobility.
However, they are generally focused on global and general
mobility patterns: through the analysis of GSM and other types
of data describing travels with different transportation means,
researchers revealed that our movements are not random, but
follow their own laws. Since such laws are very general, they
do not necessarily capture phenomena related to specific types
of mobility. To clarify this point, let us consider movements by
bike. Bikes are convenient and efficient transportation means
to use within a city, but they are not suitable to cover very
large distances. For this reason, while the accuracy in the
predictability of bikers could not differ so much from the
general pattern, the variability with respect to the characteristic
traveled distance is presumably much lower, leading to a
different mobility pattern.
The aim of this paper is to compare general mobility
with mobility by car, trying to answer the following question:
What are the differences between general mobility patterns and
patterns of car travel? To address this question, we exploit a big
mobile phone dataset collected by a European mobile phone
carrier and a GPS dataset consisting of the detailed spatio-
temporal trajectories of travels performed by cars in Italy. By
exploiting these data, we show the difference between global
mobility and mobility by car in two important aspects: the
distribution of radius of gyration and the distribution of time
spent in the visited locations.
The rest of the paper is organized as follows: Section II de-
scribes the dataset and highlights the main difference between
GSM and GPS data. Section III briefly describes the individual
mobility measures we used to unveil the patterns, while
Section IV presents a comparison between GSM and GPS
patterns. Finally, Section V concludes the paper, providing
some conclusions.
II. DATA DES CRI PT ION : GSM VS GPS
Mobile phones are nowadays very common technological
devices offering a good proxy to capture individual trajectories.
Indeed, each time a user makes a call the carrier records the
tower that communicates with the phone, effectively pinpoint-
ing users’ location. Unfortunately, this information is not ter-
ribly accurate because an individual could be anywhere within
the tower’s reception area, which can span tens of square
kilometers. Furthermore, the location is usually recorded only
when a person uses her phone, providing little information
about the whereabouts between calls. Since call patterns are
bursty [8], for most of the time the actual position of a user
is unknown.
In the current study, we exploit a GSM dataset collected by
a European mobile phone carrier for billing and operational
purposes. It contains temporal (date and time) and spatial (the
cell phone tower’s coordinates) information of all calls and
text messages sent by 3million costumers1. Table I shows an
1to guarantee anonymity, each user is identified with an anonymized security
key.
example of phone records. In order to select the most reliable
users for our purpose, we restricted our period of observation
to three months and applied some filters to the data. For each
user, we discarded locations visited only once during the entire
period of observation, and those with a number of calls less
than 0.05% of the total2. Then, from the resulting dataset we
deleted all users who visited only a single location, and those
with a call frequency less than twelve calls per day on average
during the period of observation. The filtering phase resulted
in a final dataset of 67,000 active users.
Timestamp Coordinates Caller Callee Type
2008/04/01 - 23:45:00 (32.567,2.642) A45J23 F45J23 SMS
2008/04/02 - 06:02:10 (33.282,2.221) K65232 V56YT4 Call
... ... ... ... ...
TABLE I. EXAMPLE OF PHONE RECORDS IN THE GSM DATAS ET.
Unlike GSM data, GPS traces provide high resolution
location data, storing the geodetic coordinates with an average
sampling rate of few seconds. Even though these features
are ideal in making a refined statistical analysis of human
mobility patterns, relatively few works in literature are based
on GPS data, mainly due to the difficulty to obtain complete
traces covering movements along the whole day. In this work,
we have access to a GPS dataset that stores information of
approximately 9.8Million different car travels from 159,000
cars tracked during one month (May 2011) in an area of
250km×250km in central Italy. The GPS traces are collected
by Octo Telematics Italia Srl3, a company that provides a data
collection service for insurance companies. Since GPS data
do not provide explicit information about visited locations, we
assigned each origin and destination point of the travels to
the corresponding Italian census cell, according to information
provided by the Italian National Institute of Statistics4. After
such aggregation, many users are found to have only one
visited location. We discarded them and took into account
only those users with the most frequent location (most likely
their home or work) inside the region of Tuscany. These
filtering operations produced a dataset of 46,121 users, where
a travel is described by a timestamp and a pair of coordinates
corresponding to the centroids of the origin and destination
cells of the travel (Table II).
Timestamp Origin Destination Car
2011/05/12 - 08:31:20 (32.567,2.546) (32.7,2.511) F45J23
2011/05/24 - 17:53:08 (32.1982,2.333) (33.123,2.31) H2705L
... ... ... ...
TABLE II. EX AM PLE O F RE CO RD S IN T HE GPS DATAS ET.
Table III summarizes a few properties of the two datasets
described above. It is worth noting that GPS data provide us
information about displacements performed by car only. For
this reason, we have a partial knowledge about the whole
mobility of individuals. Conversely, GSM data may provide
information about travels made using all transportation means,
although they are actually recorded only when a user calls
before and after the trip.
2this means that a location iis deleted if ni/N < 0.005, where niis the
number of calls performed in the tower i, and Nthe total number of calls
performed by the user.
3http://www.octotelematics.it
4http://www.istat.it
Dataset Volume Conveyance Precision
GPS 46,121 users Cars High
GSM 67,000 users Many Low
TABLE III. SU MM ARY OF P RO PERT IE S OF O UR D ATASET S.
III. MOBILITY MEASURES
In order to explore the statistical properties of the mobility
patterns, for each user we computed several individual mobility
measures.
The center of mass rcm of a user represents the pivot of her
individual mobility. Mathematically, it is a two-dimensional
vector representing the weighted mean of the visited locations:
~rcm =1
W
L
X
i=1
wi~ri(1)
where Lis the total number of distinct towers/cells visited
by the user/car; ~riis a two-dimensional vector representing
the geographic coordinates of tower/cell i;wiis the weight
assigned to location i; and Wthe sum of the weights over all
locations. Depending on the measure considered to evaluate
the weight of a location, two different centers of mass can be
defined. The frequency-based center of mass weights locations
according to their visitation frequency, hence wiis the number
of calls/arrivals performed in location i. In the time-based
center of mass we take wias the total time spent by the user
in location i.
Another interesting measure of an individual’s central position
is the most frequent location L1, i.e. the location where she can
be located with the highest probability, which is most likely
the home or work place. Such measure can be computed in a
very straightforward way by simply taking the tower/cell from
which the user performs the highest number of calls/arrivals.
The radius of gyration of a user [3], [4] is a mobility
measure representing the characteristic distance traveled by
each individual. It is a concept borrowed from physics, defined
as the root mean square of the weighted sum of all locations’
distances from the center of mass:
rg=v
u
u
t
1
W
L
X
i=1
wi(~ri~rcm)2(2)
where ~rcm is the vector of coordinates representing the center
of mass. We computed two types of radius of gyration: i) rg
with respect to the frequency-based center of mass, i.e. wiis
the total number of calls/arrivals in i; ii) rgwith respect to the
time-based center of mass, i.e. wiis the total time spent in i.
IV. RES ULTS
In this section, we compare the patterns found on the two
datasets, highlighting the main differences between general
mobility and mobility by car.
The first aspect we investigated is the difference between the
frequency-based radius of gyration and the time-based one.
In other words, how does the choice of locations’ weight
influence the value of the radius of gyration? Figure 1 shows
the scatter plots of frequency-rgversus time-rg, for GSM
(left) and GPS (center) users. For a better understanding of
Fig. 1. Scatter plots of frequency-rgversus time-rgfor GSM (left) and GPS (center) users. A zoomed version of the GSM scatter plot is proposed on the
right. Error bars use bin size of 50 km (left) and 5 km (center and right).
Fig. 2. Frequency- and time-based probability distributions of rgfor GSM (left) and GPS (center) users. On the right, a GSM plot for users with the most
frequent location L1in a region with size comparable to Tuscany is provided (only locations inside the region are considered in the computation of the radii).
the underlying correlation, also error bars are drawn, with bin
size of 50km (GSM) and 5km (GPS). At a first glance, in
both cases the measures seem to be correlated, as expected.
However, from a closer examination an interesting difference
emerges: while in the GSM case the mean of the error bars
tends to be biased toward the time-rg, in GPS data the mean
frequency-rgtends to be higher. One possible interpretation of
the phenomenon is that car visitation frequency of locations
distant from the center of mass is higher, leading to a bigger
characteristic traveled distance.
Figure 2 (left) shows the distributions of frequency- and time-
based radii of gyration computed on the GSM dataset. There
is no significant difference between the two curves, which
practically coincide. Conversely, a sharp difference clearly
emerges from the distributions of radii in the GPS case (Figure
2, right). Indeed, the time-based distribution is shifted towards
shorter radii, and peaks at 2km instead of 5km. This aspect
confirms the prominent role of frequency with respect to time
observed in Figure 1 (center), suggesting that cars are usually
parked for a long time in locations close to the center of mass
(like home and work locations). This effect is absent in GSM
data because people can continue their call activity even while
being stationary. Another difference we can notice is that while
the GSM curves decrease over the entire range, GPS radii show
a growing value up to 2km. This is presumably due to the
tendency of covering those small distances by foot, bike, or
bus, resulting in a lower probability to find such travels in the
GPS dataset.
To test at what extent the differences in the distribution of
rgare due by the geographic scale (GSM data refers to a
whole country, while GPS to a single region), we computed the
rgdistribution of those GSM users having the most frequent
location in a region of the country of a size and population
comparable to Tuscany. In the computation of the radius, only
the locations within the region are taken into account. As
Figure 2 (right) shows, the distribution of the radius does not
change significantly, suggesting that the shape of the curves,
and their slopes, are rather independent from the scale and are
related to the portion of mobility they represent.
The time spent across the visited locations is another inter-
esting mobility aspect it is worth investigating. Figure 3 (left)
shows the GSM distribution of time spent for the five most
frequent locations L1, . . . , L5. As we can see, the time spent
is clearly unbalanced in favor of the most important location
L1. This is reasonable, because the most frequent location
usually corresponds to user’s home or work place, which are
the locations where an individual spends most of the time.
The plot also suggests that time is proportional to frequency:
the more a user visit a location, the more time she spends
there. Such phenomenon is confirmed by Figure 4, where the
correlation, though not perfectly linear, is evident.
The same pattern is also observed on GPS data, although the
difference between L1and the other locations is less sharp
(Figure 3, right). It is worth to note that in both plots, L1
intersects the other curves approximately at the same points.
This is very interesting because, independently from the geo-
graphical scale and from the portion of mobility considered,
beyond a certain fraction of time is much more likely for a
user to be located in the L1than all the other locations.
Fig. 3. Distribution of fraction of time spent in the five most frequent locations L1,...,L5for the GSM (left) and the GPS (right) datasets.
101
102
103
104
100101102103104105
t, hours spent
c, number of calls
L1
L2
L3
L4
L5
L6
L7
L8
L9
Fig. 4. Correlation between calls and time spent in the nine most frequent
locations (GSM dataset).
V. DISCUSSION AND CONCLUSION
Our data-driven analysis showed the difference between
general mobility and mobility by car regarding two main
aspects: the distribution of radius of gyration and the distri-
bution of time spent in the visited locations. We discovered
that, regardless the geographic scale, the shape of the rg
distribution is different since mobility by car tends to poorly
cover displacements within small distances. However, in both
cases the distribution of time spent in visited locations present
a clear dominance by the most frequent location L1, with such
dominance more pronounced in the GSM dataset. Moreover,
the L1curve intersects the others approximatively at the same
points in both cases, hinting the presence of a general pattern
underlying the phenomenon.
ACKNOWLEDGMENT
The authors wish to thank the company Octo Telematics
Italia Srl for providing the car travels GPS data.
The research reported in this article has been partially sup-
ported by European FP7 project DATASIM (http://www.
datasim-fp7.eu/).
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