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Poul Greibe 1
DETERMINATION OF BRAKING DISTANCE AND DRIVER BEHAVIOUR BASED
ON BRAKING TRIALS.
Submission date: July 27, 2007
Revised: November 5, 2007
Word count: 4,839 (text) + 10 tables/figures = 7,339 words
Poul Greibe
Senior Consultant
Trafitec
Diplomvej 376
2800 Lyngby
Denmark
Phone: +45 2524 6734
Fax: +45 8870 8090
E-mail: pgr@trafitec.dk
Poul Greibe 2
ABSTRACT
Stopping sight distance is an important design parameter which influences e.g. geometric
design, road safety, construction cost. The stopping distance consists of the reaction distance
and the braking distance. The recommended braking distances in the Danish Road Standards
and Guidelines are in the main based on earlier American findings. In order to be able to
assess the validity of the recommended braking distances in relation to contemporary vehicles
and motorists in Denmark, the Danish Road Directorate has conducted a study designed to
shed light on braking behaviour and braking distances among ordinary (non professional)
motorists, at different speeds.
This was done through a measurement programme in which 22 test drivers performed braking
manoeuvres at different speeds (80, 110 and 130 km/h). Both professional and non
professional test drivers were used in the trials. Two different recent cars with ABS brakes
were used as test cars. The braking manoeuvres were carried out on dry and wet road on three
test tracks with different friction. The measurement program consisted of a total of 172
emergency stops and 23 comfort braking manoeuvres.
For professional test drivers, a good correlation was found between braking distance on wet
road, friction and initial speed. The braking distances for non professional test drivers were in
average 20-30% longer compared to the professionals, but also showed large variations.
Based on the findings plus knowledge concerning the significance of other parameters for
braking distance (condition of tyre, vehicle, etc.), we have sought to establish a new set of
recommended braking distances.
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1. INTRODUCTION
The Danish Road Directorate has conducted a study designed to shed light on braking
behaviour among non professional drivers and their braking distance at different speeds (1).
The study will be included in an assessment of the current values for braking distances
recommended in the Danish Road Standards and Guidelines.
1.1 Background and Object
Braking distance is applied as a significant basic parameter in e.g. calculations of stopping
sight distance. A vehicle’s braking distance depends on a number of factors pertaining to the
vehicle, the road and the driver’s behaviour. The most important factors are:
• speed
• coefficient of friction
• braking behaviour/technique
• braking system and condition
• tyre condition
• road’s vertical grade
All these factors affect braking distance to a greater or lesser extent depending on the actual
conditions when decelerating. A general method for determining braking distance at different
speeds, which is representative for the composition of cars, drivers and friction, requires
knowledge of the significance of the individual factors for overall braking distance.
The purpose of this study is to assess the braking behaviour of non professional drivers,
including braking distances under different physical conditions. The findings will be included
in an assessment of the present methods for calculating braking distance in the Danish Road
Standards and Guidelines which is essentially based on early American findings.
1.1.1 Methods to estimate braking distance
The present method, as described in the Danish Road Standards and Guidelines (2), is based
on measurements of friction values for tyre/roadway and the physical laws of deceleration.
Here the braking distance is obtained from the speed, coefficient of friction and the roadway
grade by applying the following formula:
2
2
6.3)(2 ⋅+⋅⋅
=
sg
V
l
brake
brake
µ
l
brake
= braking distance (m)
V = speed (km/h)
g = acceleration due to gravity (9.81 m/s
2
)
µ
brake
= mean coefficient of friction
s = roadway grade
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The recommended friction values and the calculated braking distance (on level road) from the
current Danish Road Standards and Guidelines are shown in Table 1. It should be noted that
the Danish Road Standards and Guidelines often operate with an extra safety margin of +20
km/h when determining the braking distance.
In the latest version of the AASHTO Green Book from 2001 (3), determination of the braking
distance for use in calculating stopping sight distances has been altered from the more
traditional calculation method using coefficients of friction (like the Danish) to a calculation
method based on behavioural recordings and measurements from braking trials. Based on the
findings of a large-scale measurement programme indicate that by far the majority of all
motorists brake with a deceleration of more than 3.4 m/s
2
(4). By applying 3.4 m/s
2
, we obtain
braking distances as shown in Table 1. As can be seen, some differences in the Danish and
Green Book values are found, especially at high speed levels.
TABLE 1 Coefficients of friction and braking distance from the Danish Road
Standards and Guidelines and braking distances from AASHTO Green Book
Danish Road Standards and Guidelines
AASTHO Green
Book Speed
(km/h)
Coefficient of
friction
Braking distance
(m)
Braking distance
(m)
50 0.38 26 29
60 0.36 39 41
70 0.35 55 56
80 0.34 74 73
90 0.33 97 93
100 0.31 127 115
110 0.30 157 139
120 0.29 195 165
130 0.28 234 194
2. MEASUREMENT PROGRAMME
In order to investigate braking distances and driver behaviour a measurement programme in
which 22 test drivers (professional and non professional drivers) performed braking
manoeuvres at different speeds (80, 110 and 130 km/h) was conducted. The professional test
drivers were chosen among trained drivers from the traffic police. Two different recent cars
with ABS brakes were used as test cars. (More than 90% of cars in Denmark are equipped
with ABS brakes). The braking manoeuvres were carried out on three test tracks with
different friction. Separate road sections on each test tracks were used for braking trials on dry
and wet road surface respectively. The majority of the manoeuvres performed were
emergency stops, in which the test driver was required to bring the vehicle to a complete
standstill as quickly as possible. In addition, a small number of comfort braking manoeuvres
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were performed in which the test driver was required to bring the vehicle to a comfortable
stop.
A total of 172 emergency stops and 23 comfort braking manoeuvres were performed. Two
highway roads (closed for traffic during braking trials) and one closed airfield running
taxiway were used as test tracks. Separate road sections (on each test track) were used for wet
and dry braking trails. The friction on the test tracks was measured by the Danish Road
Institute’s measurement vehicle. Table 2 shows the recorded friction values at 60 km/h and
20% slip (standard measurement method).
The number of test drivers at each test track can also be seen in Table 2. Every test driver had
to make emergency stops in both cars at 80, 110 and 130 km/h on dry and wet road
respectively. However, not all non professional test drivers performed emergency stops from
130 km/h. Several drivers did not feel confident performing the manoeuvre at this speed, in
which case it was omitted in the measurement programme.
TABLE 2 Measured friction and number of test drivers at each track
Measured Friction No of test drivers Test track
Wet brake
road section
Dry brake
road section
Professional Non professional
1 0.49 0.49 2 2
2 0.52 0.64 2 4
3 0.75 0.74 2 10
2.1 Measurement equipment
In order to record braking behaviour, a measurement wheel was mounted on the car, a
pressure sensor was fitted to the brake pedal, and a notebook PC was installed for data
collection. During the braking trials, data on distance travelled, time code and the recorded
pressure on the brake pedal were logged and stored on the PC. Based on these data, it was
possible to calculate speed, deceleration, etc.
2.2 Test cars
For the braking trials, two cars were used: a small and a medium-sized car, which were
judged to be fairly representative of recent makes of car in Denmark. The cars were a Fiat
Grande Punto and an Opel Vectra. Both cars had ABS brakes and manual transmission. The
cars were fitted with new summer tyres (Continental EcoContact 3) prior to the first braking
trials. The tyres were however ’run in’ by 500 kilometres’ ordinary driving prior to the first
measurement day. The tyres used were judged to be average, typical summer tyres with
medium to good braking capability according to tyre tests (5).
The same tyres were used for all the braking trials. The tread depth from the start was
measured as 7-8 mm and the total wear during the entire programmes was recorded as < 1
mm.
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2.3 Test drivers
On each of the three test tracks, both professional and non professional test drivers performed
braking manoeuvres. The results from the professional test drivers were assumed to be able to
describe the optimal emergency stop under the given conditions. The non professional test
drivers (a total of 16) were mainly recruited among staff in the Danish Road Directorate. They
consist of 11 men and 5 women, mainly in the age-range 25-39. The results from the non
professional test drivers are intended to shed light on the individual differences in braking
behaviour presented by ordinary drivers.
2.4 Water truck for wet surface
The wet road surface was achieved with the aid of a water truck, which dispersed water onto
the braking section immediately before each braking trial. The calculated volume dispersed by
the truck was 1.3-1.6 litres/m
2
(water depth = 1.3-1.6 mm). Due to the road’s cross slope,
some of the water would naturally have run off the road again before the trial was conducted.
Typically it took a couple of minutes from the water truck dispersing the water until the
braking trial was performed.
3. FINDINGS FROM BRAKING TRAILS
3.1 Results for professional drivers
The average braking distances (L
brake
) for professional test drivers at 80, 110 and 130 km/h on
the three test tracks are shown in Table 3. Car 1 and Car 2 values are combined since the
difference were negligible. As will be seen, the difference in L
brake
on dry road is
insignificant, but for wet road is somewhat larger.
TABLE 3 Average L
brake
by speed, test track and dry/wet road surface
Test track 1 Test track 2 Test track 3
Speed
dry wet Dry wet dry wet
80 km/h 30 m 35 m 30 m 34 m 30 m 29 m
110 km/h 55 m 64 m 55 m 61 m 55 m 55 m
130 km/h 76 m 88 m 76 m 84 m 76 m 76 m
The average deceleration (Dec
brake
) for professional test drivers, based on the entire braking
run has been analysed. Overall, the difference between the two cars was small. Dec
brake
was
measured as 8.4 m/s
2
for dry road and 7.9 m/s
2
for wet road. The difference between the 3 test
tracks must be attributable to differences in friction (discussed later). Further, higher
deceleration values are seen at higher initial speeds, which is due to the fact that braking was
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more effective at higher speeds, since the braking initiation time accounts for a smaller
proportion of the total braking time at higher speeds.
Figure 1 shows the average deceleration values for speed intervals 120-100, 100-70, 70-50
and 50-30 km/h for the professional test drivers. The deceleration values are generally higher
on dry compared with wet road. Furthermore, the deceleration values are higher within the
low speed intervals compared with the high speed intervals. On dry road, Dec
50-30
was approx.
9.5 m/s
2
, while Dec
100-70
was approx. 9.1 m/s
2
. In addition, there is some difference in the
measured deceleration values depending on whether braking started from 130, 110 or 80
km/h. For example, braking from 80 km/h has the poorest Dec
50-30
, while breaking at 110 or
130 km/h is more effective in the shape of higher deceleration values. The reason for this may
be that an extended braking trajectory, from e.g. 130 km/h, raises tyre and brake temperature,
which in turn boosts braking capability.
7
7.5
8
8.5
9
9.5
10
Dec120-100 Dec100-70 Dec70-50 Dec50-30
Deceleration values by speed interval
Deceleration (m/s2)
80 - Dry
110 - Dry
130 - Dry
80 - Wet
110 - Wet
130 - Wet
130 - Dry
110 - Dry
80 - Dry
130 - Wet
110 - Wet
80 - Wet
FIGURE 1 Average deceleration values (m/s
2
) on dry and wet road, by speed intervals
and initial speed.
3.1.1 Braking distances and friction
Figure 2 shows the measured braking distances (L
brake
) for 80, 110 and 130 km/h,
respectively, on dry and wet road with different friction values (professional test drivers
only). On dry road, L
brake
is almost constant for friction values in the range 0.5 – 0.7. On wet
road, we see gradually increased braking distances the lower the friction. This is true of all
speeds. At 130 km/h, L
brake
for example, increases from 76 m to 91 m when friction is reduced
from 0.75 to 0.5. This corresponds to an increase in L
brake
of approx. 20%. The same
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percentage increase in L
brake
is also seen in the case of the other speeds. It should also be
noted that, at high friction, L
brake
is the same for dry and wet road.
The corresponding values for Dec
brake
are also shown in Figure 2. Dec
brake
for dry road is at 8-
8.5 m/s
2
irrespective of friction, while for wet road, it drops from approx. 8.5 m/s
2
to approx.
7 m/s
2
, when the friction is reduced from 0.75 to 0.5.
Deceleration and friction
0
1
2
3
4
5
6
7
8
9
10
0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80
Friction (60kmt - 20% slip)
Decbrake - corr (m/s2)
130 - Dry
130 - Wet
110 - Dry
110 - Wet
80 - Dry
80 - Wet
Braking distance and friction
0
10
20
30
40
50
60
70
80
90
100
0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.8
Friction (60kmt - 20% slip)
Lbrake - corr (m)
Deceleration and friction
0
1
2
3
4
5
6
7
8
9
10
0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80
Friction (60kmt - 20% slip)
Decbrake - corr (m/s2)
130 - Dry
130 - Wet
110 - Dry
110 - Wet
80 - Dry
80 - Wet
Braking distance and friction
0
10
20
30
40
50
60
70
80
90
100
0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.8
Friction (60kmt - 20% slip)
Lbrake - corr (m)
FIGURE 2 Braking distance (L
brake
) and deceleration (Dec
brake
) as a function of friction.
A regression analysis to describe Dec
brake
as a function of friction and initial speed was carried
out. This was done using the same model as in (6), where deceleration is described using a
function in which the square root of the friction is used.
The model is shown below:
0
VbaDec
fricbrake
⋅+⋅=
µ
where
Dec
brake
is the average deceleration for the entire braking run (m/s
2
)
µ
fric
is the recorded friction on the test track at 60 km/h – 20% slip
V
0
is the initial speed before braking (m/s)
a and b constants found by regression
For the recorded deceleration values on wet road (33 observations), the following results were
obtained:
97.0028.079.8
2
0
=⋅+⋅= RVDec
fricbrake
µ
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This allows L
brake
to be calculated using various friction values by the formula:
)028.079.8(2
2
0
2
0
2
0
V
V
Dec
V
L
fric
brake
brake
⋅+⋅⋅
=
⋅
=
µ
The calculated values for Dec
brake
and L
brake
using the formulas is shown for 80, 110 and 130
km/h in Table 4.
TABLE 4 Estimated deceleration (m/s
2
) and braking distance (m) for wet road. Based
on regression analysis.
80 km/h 110 km/h 130 km/h
Friction
Dec
brake
(m/s
2
)
L
brake
(m)
Dec
brake
(m/s
2
)
L
brake
(m)
Dec
brake
(m/s
2
)
L
brake
(m)
0.4 6.2 40 6.4 73 6.6 99
0.5 6.8 36 7.1 66 7.2 90
0.6 7.4 33 7.7 61 7.8 84
0.7 8.0 31 8.2 57 8.4 78
0.8 8.5 29 8.7 54 8.9 74
According to the table, a wet road surface with a friction of 0.4 will produce a Dec
brake
of 6.2-
6.6 m/s
2
depending on initial speed. According to the Danish Road Standards and Guidelines,
a friction value of 0.4 is the minimum requirement for roads in operation.
3.2 Results for non professionals
The measured braking distances in emergency stops show, that non professional test drivers in
average have longer braking distances compared to the professionals’ for all speeds and
dry/wet conditions. At the same time, we see a larger spread in braking distances for the non
professional test drivers. In addition, we find that higher speed produces higher spread.
Several test drivers brake on a par with the professionals, and some even have minimum
values that are better than those of the professionals. But there are also test drivers whose
average braking distance is more than 40-50% longer than the professionals, and in some
cases exceeds 100%.
To analyses this in more detail, we calculated the L
brake%
which is the percentage difference in
braking distance for the non professional test driver in relation to braking distance for the
professional under the same conditions (speed, road surface, test track and car).
Overall, the L
brake%
is in the order of 20-25%, but the figures hold great variation. Figure 3
shows the individual test drivers mean value for L
brake%
. Besides, the mean, the min. and max.
values are shown. The majority of the test drivers have an L
brake%
that averages less than 20%,
but some test drivers whose average L
brake%
is more than 40-50%.
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L
brake%
of test drivers
(percentage diff. in relation to professionel test driver)
-20%
0%
20%
40%
60%
80%
100%
120%
140%
160%
180%
1 2 5 6 9 1011121314151617182122
Test driver No
Lbrake%
FIGURE 3 L
brake%
for non professional test drivers, expressed as a mean, min. and max.
value.
If we look at the recorded braking distances on wet road at 80 and 110 km/h in isolation, the
L
brake%
breaks down as follows: For 14% of all the recorded braking distances on wet road, the
L
brake%
is negative (i.e. shorter than the professionals’); for 29% the L
brake%
is between 0-10,
while for 22% the L
brake%
is 10-20. By far the majority (80%) of the braking distances have an
L
brake%
that is less than or equal to 30.
One of the parameters responsible for great variation in L
brake%
, is the number of braking trials
carried out. The more times the test drivers carried out the manoeuvre, the more effective their
braking became. In the 1st braking trial, which was always at 80 km/h on dry road, the L
brake%
averages 23%. After 5-6 trials, the majority of test drivers have tried both dry and wet road at
different speeds, and begin to feel more confident about the manoeuvre, and the L
brake%
falls.
Other results for the non professional test drivers are summarized below:
• the Dec
brake
(average deceleration for the entire braking run) was in average 7.4 m/s
2
on dry
road and 7.0 m/s
2
on wet road. This is approx. 10% less than those of the professionals
• male test drivers in the age-group 25-50 have the shortest braking distances compared to
female test drivers in the same age-group. For age-group 50-70 (all male), the longest
braking distances were found. The number of test drivers in the study is however, modest,
which means that the figures barely can be used to generalise.
Poul Greibe 11
• the L
brake%
expressed by speed, dry/wet road and car 1 and car 2 showed only minor
differences.
• on average, the pressure on the brake pedal for the whole of the braking run, was recorded
as 34.8 kg for the non professional test drivers and 74.0 kg for the professionals.
• analysis shown that deceleration drops once the pressure on the brake pedal drops below
approx. 10-15 kg. However, there is no consistent difference in deceleration once the
pressure exceeds 10-15 kg.
• on average, the time it takes from the pedal being touched until the pressure reaches at least
10 kg, was recorded as 0.83 sec. for the non professional test drivers and 0.05 sec. for the
professionals.
• braking on wet road and at high speeds, is the situations in which the non professional test
drivers brake most tentatively
• in the comfort braking trials, were non professional test drivers were required to bring the
vehicle to a comfortable stop, the test drivers had an average deceleration of 3.2 m/s
2
in the
speed interval from 70-20 km/h.
4. THE SIGNIFICANCE OF OTHER PARAMETERS FOR BRAKING DISTANCE
The measurement programme contained a selection of the parameters of significance for
braking distance. Other important parameters that were not included in the measurement
programme are (among others):
• Make of tyre
• Summer tyres versus winter tyres
• Tyre tread depth
• Make of car
• Loaded or non-loaded car
Information about the significance of the listed parameters for braking distance is based on
literature and is summarised in Table 5. The table shows the effect on braking distance on wet
road for the various parameters compared with either an average state or an altered state.
TABLE 5 Effect on braking distance on wet road.
Parameter
Effect in
relation to:
Effect %
[min/max]
Average in
relation to
measurement
programme
Effects based on
literature:
Make of tyre Average tyre -10% - +10% +0% (7), (8), (9)
Winter tyre Summer tyre +5% - +35% +15% (7)
Tread depth 1.6 mm 8 mm +0% - +50% +25% (6), (9), (10), (11)
Make of car Average car -10% - +10% +0% -
Loaded Non-loaded -10% - +15% +4% (6), (12)
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Winter tyres for example have a braking distance +5% to +35% longer than summer tyres.
This puts the average at 15%. In relation to our measurement programme, which was
conducted on summer tyres, the braking distance thus has to be increased by an average of
15% to be applicable to vehicles with winter tyres.
The question is whether the effects on braking distance can indeed be summed. A poor
vehicle with poor tyres that are loaded will, if the effects are summed, risk producing a
10%+35%+50%+10%+15% = 120 % longer braking distance compared with those observed
in the measurement programme. In the best case, the braking distance might be 25% shorter.
However, the probability of finding a vehicle with these extreme min./max. values is
presumably very small. Overall, the conclusion is that a large spread in the braking distance is
likely, depending on the listed parameters, if one permits the listed effects to be summed. A
better description of how tyres, brakes, car makes and the interaction of these affect braking
distance would require supplementary data or further practical trials.
5. NEW RECOMMENDED BRAKING DISTANCES
An attempt is made in the following to establish a new set of recommended braking distances
for use in Denmark. This is done on the basis of the findings from the measurement
programme as well as knowledge of the different parameters’ influence on braking distance.
New recommended values for braking distances are provided on the basis of the following
considerations:
• the braking distance should reflect worst-case-scenario road conditions, which equate to wet
road with low friction. Low friction is set at 0.4, which is consistent with friction
requirements for roads in operation. Wet road is assumed to be in the same state as that
during the measurement programme, i.e. clean, but with a water membrane of approx. 1
mm.
• the braking distance should reflect the braking capabilities of a vehicle whose braking
capabilities are at the weak end of the scale among ordinary cars, but which otherwise
conforms to legal brake, tread pattern requirements, etc.
• the braking distance should reflect the braking behaviour found among the worst performing
drivers (among non professional) travelling on the roads.
• the braking distance assumes that the vehicle is fitted with ABS brakes.
The braking distance for wet road with friction of 0.4 is determined from the results in
Section 3.1.1. Here we find that a professional test driver is able to achieve a Dec
brake
of
approx. 6.5 m/s
2
under these conditions (extrapolated from the recorded data).
By far the majority of the non professional test drivers produced braking distances 0-20%
longer than the professionals’ (see Section 3.2). It is assumed that the weakest half of the non
professional drivers have a braking distance 30% longer than the professionals.
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The braking distance for a legal vehicle in which the braking capability is poor due to worn
and poor tyres, poor brakes etc. is set (rounded figures) at 45% longer than the observed
braking distances for the test cars used in the measurement programme.
Overall, this results in recommended braking distances as shown in the Table 6.
TABLE 6 New recommended braking distances. Based on wet and clean road (on level)
with a friction of 0.4.
80 km/h 110 km/h 130 km/h
L
brake
– professional in test car 40 m 73 m 99 m
Behavioural increment (+30%) 12 m 22 m 30 m
Vehicle increment (+45%) 18 m 33 m 45 m
Recommended L
brake
(sum) 70 m 128 m 174 m
The recommended braking distances correspond to an average Dec
brake
of 3.7 m/s
2
.
Figure 4 shows the new recommended braking distances compared with the existing ones
from the Danish Road Standards and Guidelines and the Green Book, together with those
recorded in the measurement programme.
As shown by Figure 4, the new recommended braking distances are almost identical with the
current ones for speeds below 90 km/h. For higher speeds, e.g. 130 km/h, the new
recommended braking distances are approx. 25% shorter than the current ones. In relation to
the current recommendations, incl. safety increment, the braking distance for 130 km/h is
approx. 50% shorter. In relation to the braking distances from the Green Book, the new
recommended braking distances are approx. 5-10% shorter.
The method for determining the new recommended braking distances based on the findings of
this study calls for a few accompanying remarks:
The method for determining the vehicle increment of 45% is somewhat uncertain. Section 4
describes the significance of various parameters for braking distances. If these are summed
uncritically, we find large spreads in the braking distance of vehicle with better or poorer
braking capability (in relation to the test cars). The vehicle increment is set at 45%, based on a
law of averages combined with what ”would appear fairly reasonable”, but in reality, the
vehicle increment is not precisely known.
It is also debatable whether the recommended braking distance should be based on tyres that
are only just compliant with the statutory requirement of 1.6 mm tread depth, or whether
stricter requirements for tread depth would be preferable (e.g. 3 mm). The key factor in the
vehicle increment is tyre tread depth.
In determining new recommended braking distances, the vehicle increment and behavioural
increment were summed uncritically. It is debatable whether the recorded braking distance
Poul Greibe 14
can be translated directly to another vehicle with poor braking capability. One might, for
example, imagine that the difference between a professional test driver and a non professional
test driver would not be the same if the braking was performed in a vehicle with very poor
braking capability. This then would make it incorrect to sum the vehicle increment (45%) and
the behavioural increment (30%).
Overall, the position is that the new recommended braking distances have been determined
somewhat “cautiously” with a good safety margin in relation to the cars and motorists driving
on Danish roads.
Braking distance for respectively:
Measured in programme, Danish Guidelines, Green Book and new recommended values
0
50
100
150
200
250
300
350
40 50 60 70 80 90 100 110 120 130 140
Speed (km/h)
Lbrake (m)
Measured L
brake
- fric=0.49
Measured L
brake
- fric=0.64
Measured L
brake
- fric=0.74
Estimated L
brake
- fric=0.4
Current Danish Guidelines
Behavioural increment (+30%)
Vehicle increment (+45%)
New recommended braking distance
Green Book
Current Danish Guidelines +
safety increment
FIGURE 4 Recorded values for L
brake
and new and existing recommended braking
distances
6. SUMMARY
The current recommended braking distances in the Danish Road Standards and Guidelines are
in the main based on earlier American findings. In order to be able to assess the validity of the
recommended braking distances in relation to contemporary vehicles and motorists in
Denmark, the Danish Road Directorate has conducted a study designed to shed light on
Poul Greibe 15
braking behaviour and braking distances among ordinary (non professional) motorists, at
different speeds.
This was done through a measurement programme in which 22 test drivers performed braking
manoeuvres at different speeds (80, 110 and 130 km/h). The majority of the test drivers who
participated were recruited from among non professional drivers. However, 6 out of the 22
test drivers were professional test drivers with extensive experience in advanced driving
technique. Two different recent cars with ABS brakes were used as test cars. The braking
manoeuvres were carried out on dry and wet road on three test tracks with different friction.
The majority of the manoeuvres performed were emergency stops, in which the test driver
was required to bring the vehicle to a complete standstill as quickly as possible. In addition, a
small number of comfort braking manoeuvres were performed in which the test driver was
required to bring the vehicle to a comfortable stop.
Based on findings of the measurement programme (172 emergency stops and 23 comfort
braking manoeuvres) and knowledge obtained concerning the significance of other parameters
for braking distance (choice of tyre, vehicle, etc.), we have sought to establish a new set of
recommended braking distances. These are based on “worst case scenarios”, i.e.
• wet road surface with poor friction (friction=0.4 – minimum requirement for roads in
operation)
• driver with tentative braking behaviour
• vehicle with poor braking capability
The new recommended braking distances for speeds of 80, 110 and 130 km/h are 70 m at 80
km/h, 128 m at 110 km/h, 174 m at 130 km/h.
The braking distances are assessed as somewhat ”cautiously determined” with a good safety
margin in relation to the cars and motorists driving on Danish roads.
For speeds below 90 km/h, the new recommended braking distances are almost identical with
both the existing ones in the Danish Road Standards and Guidelines and with Green Book. At
greater speeds, e.g. 130 km/h, the new recommended braking distances are approx. 25%
shorter than the existing ones and 10% shorter compared to Green Book (see Figure 4).
Poul Greibe 16
7. REFERENCES
1.
Braking distance, friction and behaviour. Findings, analyses and
recommendations based on braking trials.
Poul Greibe, Trafitec Report, July 2007
2.
Forudsætninger for den geometriske udformning. Veje og stier i åbent land –
Hæfte 1. Vejregelrådet, 1999 (Danish Road Directorate)
3. A policy on geometric design of highways and streets. Green Book
AASHTO, 2001
4. Determination of Stopping Sight Distances.
Fambro, D.B., K. Fitzpatrick, and R.J. Koppa, NCHRP Report 400. TRB,
National Research Council, 1997
5.
Dæktest
Motor 4/2005, Motor 4/2006 (Motor magazine)
6.
Mögliche Bremsverzögerung in Abhängigkeit von der Griffigkeit.
Ralf Roos and Matthias Zimmermann. Universität Karlsruhe, 2004
7.
Sommerdæks og vinterdæks bremseegenskaber ved lav temperatur mv.,
forskningsresultater og målinger.
Færdselsstyrelsen (Denmark’s Road Safety and Transport Agency), 2006
8.
Sommerdæk på glatis
Motor 5/2001 (Motor magazine)
9.
Personbilsdäcks bromsfriktion på våt asfaltbeläggning
Nordström and Gustavsson. Swedish National Road and Transport Research
Institute, report 61-1996, 1997
10.
Tyre Safety Campaign
Tyre Specialists of Finland, 2005
(Interest organisation for tyre companies)
11.
Continental’s website
http://www.conti-online.co.uk/generator/www/uk/en/
continental/portal/general/safety/3mmtread_en.html (seen February 2007)
12.
Consumer Braking Information
Final report – prepared by U.S. Army Aberdeen Test Center
National Highway Traffic Safety Administration (NHTSA), 2003
