At normal pedal braking on split-μ a driver can actively steer or adjust brake level to control lateral drift. The same driver response and thus lateral deviation cannot be assumed when brakes are automatically triggered by a collision mitigation system, since the driver can be expected as less attentive. To quantify lateral deviation in this scenario a test was run at 50 km/h with 12 unaware drivers in a heavy truck. Brakes were configured to emulate automatic braking on split-μ. Results show that the produced maximum lateral deviation from the original direction was 0.25 m on average. Two drivers deviated by 0.5 m. This can be compared to 2.2 m which was reached when steering was held fixed

Jacobson, Bengt

Laine, Leo

Tagesson, Kristoffer

Chalmers Publication Library

Title: Driver Response to Automatic Braking under Split Friction Conditions 
 
Publisher: JSAE 
 
In proceedings of: the 12th International Symposium on Advanced Vehicle Control 
 
Copyright: © 2014 Society of Automotive Engineers of Japan, Inc. All rights reserved 
 
Version: Accepted article 
 
To cite this article: TAGESSON, Kristoffer; JACOBSON, Bengt; LAINE, Leo. Driver Response to 
Automatic Braking under Split Friction Conditions. In: 12th International Symposium on Advanced 
Vehicle Control Proceedings. Tokyo, 2014. p. 666-671. 
 
AVEC ’14 
Driver Response to Automatic Braking under Split Friction 
Conditions 
  
Kristoffer Tagesson1,2, Bengt Jacobson1, and Leo Laine1,2,  
1Division of Vehicle Engineering & Autonomous Systems, Chalmers University of Technology 
2Department of Chassis Strategies & Vehicle Analysis, Volvo Group Trucks 
Technology 
 
Division of Vehicle Engineering & Autonomous Systems,  
Chalmers University of Technology, 
 Göteborg, SE-412 96, SWEDEN  
 Phone: (+46) 313234867 
 E-mail: kristoffer.tagesson@volvo.com; bengt.jacobson@chalmers.se; leo.laine@volvo.com 
  
At normal pedal braking on split-μ a driver can actively steer or adjust brake level to control 
lateral drift. The same driver response and thus lateral deviation cannot be assumed when brakes 
are automatically triggered by a collision mitigation system, since the driver can be expected as 
less attentive. To quantify lateral deviation in this scenario a test was run at 50 km/h with 12 
unaware drivers in a heavy truck. Brakes were configured to emulate automatic braking on split-μ. 
Results show that the produced maximum lateral deviation from the original direction was 0.25 m 
on average. Two drivers deviated by 0.5 m. This can be compared to 2.2 m which was reached 
when steering was held fixed. 
  
Topics/Active safety and driver assistance systems, Driver modelling  
 
1. INTRODUCTION 
 
A road section with significantly different level of 
friction between left and right vehicle sides is said to 
have split friction or split-μ. Common reasons for 
split-μ are: oil spillage, uneven ice coating, and 
one-sided aquaplaning. When cruising or accelerating 
slowly the driver may not even notice the effect, but 
when braking hard in an emergency situation the effect 
from unbalanced braking forces may cause serious 
rotation of the vehicle towards the side of high friction. 
For trucks towing one or more trailer this can also lead 
to jack-knife [1]. 
At the event of modest rotation the driver can steer 
and balance uneven braking forces. However if the 
driver is surprised by the situation and thus unprepared 
it is likely that substantial lateral deviation from ego 
lane can occur before the driver has responded. This can 
result in run of road or collision with oncoming traffic. 
Fig. 1 provides an example of this where a truck ends up 
in the opposite lane.  
Furthermore a hypothesis is that when braking is 
activated by an advanced emergency braking system, 
AEBS, automatically the surprise would become even 
bigger. And thus also produce bigger lateral deviation 
and higher risk of jack-knifing. One can also note the 
similarity to front tyre blow-outs that yearly leads to 
some fatal accidents e.g. see [2]. In [3] a truck simulator 
study was performed where the left front tyre exploded. 
It was observed that driver behaviour very much 
depended on if the blow out came as a surprise or not. 
Hence it is important to include the effect from surprise 
also when investigating automatic braking under split 
friction conditions. 
 
 
Fig. 1 An example with a truck (1) that brakes because 
of a stationary car (2). A patch of one-sided low friction 
(4) causes the truck to yaw and move sideways (5). 
When adding an oncoming car (3) into the scene an 
accident is imminent. 
 
The split friction braking scenario has been a 
well-known hazard for decades and many innovations 
have been presented to reduce the effects [4]. Some 
have been proven more effective than others. E.g. brake 
pressure limiting approaches are already used on many 
vehicles, but have the effect of reducing brake 
AVEC ’14 
performance. There are also legal requirements for 
split-µ braking that limits the allowed lateral deviation, 
under certain conditions [5]. The aim of this paper is to 
study if these legal requirements together with 
commonly used functionality are enough, on split-µ, 
when AEBS is introduced and soon mandatory on heavy 
trucks in Europe [6]. Or if there is a need of further 
supporting the driver.  
In order to understand the severity in automatic 
braking under split friction conditions it is important to 
know how a driver reacts. For this reason a test was set 
up exposing drivers to a rather sudden situation, where 
the truck pulls sideways during automatic braking. 
In section 2 the arrangement of the experiment is 
described, results follow in section 3 and finally section 
4 present some conclusions. Notations and properties 
used, especially sign conventions, are compliant with 
ISO 8855 [8]. 
 
2. METHOD 
 
The test was run with a 9 ton solo tractor on a test 
track where 12 drivers were exposed to sudden 
automatic braking. Research results were obtained 
through informed consent. Brakes were controlled to 
emulate split-μ conditions on an even test-track. The 
drivers were not aware of the true purpose of the test in 
order to preserve the effect from surprise. The test was 
carefully designed to guarantee safety. 
Only professional drivers, normally driving 
durability tests of trucks, took part. The average age was 
42, the oldest driver was 60 and the youngest 27. Only 
one driver had experience from pure brake or handling 
tests. Drivers were told that the intension of the test was 
to record normal positioning in lane and that they 
should run back and forth inside a straight lane for 
300 m. Cruise control was set to 50 km/h. After running 
back and forth for 5 minutes, without any intervention, 
an operator fired of automatic braking as described. 
After the first unexpected intervention two repeated 
runs were made at the same speed, followed by two 
more at 70 km/h. 
 
2.1 Vehicle and Track 
A 6×2 Volvo FH pusher tractor was used in the 
experiment having the pusher axle lifted. Since the test 
was set up for the first time it was run without any 
trailers to ensure safety. The same goes with the 
selection of speed. It was low initially to guarantee 
safety. For more details on the vehicle used see Table 1. 
The test was run on a test track in Sweden during 
two days in December. Temperature was 3-8oC. The 
track was slightly wet, but it did not rain. No other 
vehicle was nearby; therefore cones were put in the 
adjacent lanes creating a sense of danger. Fig. 2 
provides an illustration of the set-up. 
 
 
Table 1 Vehicle System Parameters 
Description Value Unit 
Wheelbase, distance between front and 
drive axle 
4.1 m 
Vehicle width, to outer wheel side 2.25 m 
Front axle vertical load 58470 N 
Pusher axle vertical load (lifted) 0 N 
Drive axle vertical load 29430 N 
Overall steering ratio 23.2 - 
Steering wheel radius, measured from 
centre to rim edge 
0.225 m 
Wheel effective radius 0.5 m 
 
 
Fig. 2 The test was run at 50 km/h using cruise control. 
Soft cones were used to create a sense of danger. 
 
2.2 Brake Controller 
In [6] requirements implicitly say that an AEBS 
system shall be capable of performing deceleration by at 
least 2.2 m/s2 during the emergency braking phase. Also 
stated is that ''the AEBS shall be designed to minimise 
the generation of collision warning signals and to avoid 
autonomous braking in situations where the driver 
would not recognise an impending forward collision'', 
i.e. nuisance should be avoided. In practise this means 
that the automatic emergency braking phase would have 
to be triggered closer to the imminent collision. In [9] 
normal braking behaviour of drivers in cars is analysed. 
At 80 km/h and time to collision, TTC, at 2.7 s it is a 
75% chance that a driver would treat the required brake 
action as hard, in order not to collide with a moving 
target vehicle. In [10] a study on truck driver 
deceleration behaviour was made. At 80 km/h it was 
observed that normal braking does occur as late as TTC 
3.9 s. This is based on 10000 normal brake interventions 
from euroFOT data. With the combined findings in [9] 
and [10] and the requirement regarding nuisance in [6] 
an AEBS should at least be capable of decelerating at 
3.5 m/s2 during the emergency braking phase. Here 
assuming that brake initiation is delayed until TTC 4.0 s 
and that the brake system has a delay of 0.2 s from 
brake request until full deceleration is reached (this 
delay was verified on used truck). Therefore as target 
deceleration 3.5 m/s2 was used in a brake controller.  
The controller consisted of a feedforward and a 
AVEC ’14 
feedback part. The feedforward part was constant and 
the feedback part was a PI controller with integrator 
saturation and delayed initiation.  
The sum of the feedforward part and the PI 
controller was fed into a static allocation function. The 
proportion between left and right brake torque was fix 
and set to 4. This value was derived from real split-μ 
testing, using normal factory brake system settings. In 
this mode the tractor was compliant with [5] since it had 
a brake pressure limiting function setting allowed 
difference between left and right brake pressure. The 
relation between front and rear brake pressure was set 
according to static normal loads. A linear relation was 
assumed between brake pressure and brake force. In Fig. 
3 a block diagram of the controller is shown. All 
properties included are listed in Table 2. 
In case the driver pressed the brake pedal a select 
high pressure routine was used per wheel. If the driver 
pressed the accelerator pedal the test was aborted. 
 
 
Fig. 3 Block diagram of brake controller 
 
3. RESULTS  
 
All 12 trajectories relating to the very first exposure 
of automatic braking interventions are shown in Fig. 4. 
All lines have been rotated and moved so that (0, 0) m 
correspond to where the operator activated the 
automatic braking, i.e. brake onset. The stopping 
distance, counting from brake onset, ranges from 30.2 m 
to 33.3 m. Two drivers instinctively deactivated the 
intervention by pressing the accelerator pedal. This 
corresponds to the two trajectories that continue to 
travel even after 35 m. The mean maximum lateral 
deviation was 0.25±0.07 m, using 95% confidence level. 
Two drivers deviated by 0.5 m. The open loop response 
produced by locking the steering wheel, StW, is also 
shown. It deviates by 2.2 m at standstill.  
Corresponding time series are shown in Fig. 5. 
Looking at the speed curves it can again be seen that 
two drivers instinctively deactivated the intervention by 
pressing the accelerator pedal. The second subfigure 
shows StW angle relative change, calculated as: 
 
H(t) = H(t)-H(0)         (1) 
 
where H(t) is StW angle at time t. After 0.6 s, on 
average, drivers started steering. Some drivers 
responded with a smooth and steady movement of the 
steering wheel, whereas others oscillated widely.  
The negative scrub radius, which acts destabilizing, 
can be observed in the StW torque plot. Around 
-2.5 Nm of the disturbance reached the driver.  
As seen in the last subfigure yaw rate starts building 
up after 0.3 s and also the response shows a one period 
sine wave. Corresponding frequency, 0.5 Hz, happens to 
match the resonance frequency of several truck 
combination types, see [7]. This highlights the 
importance of extending the study for multi-unit truck 
combinations. 
 
Table 2 Brake Control Parameters 
 Notation Value Unit 
Feedback    
Measured 
longitudinal 
acceleration 
ax - m/s2 
First order low pass 
filter with time 
constant 0.2 s 
LP - - 
Target longitudinal 
acceleration 
ax,ref -3.5 m/s2 
PI saturation  6000 N 
P-gain  4000 N/m/s2 
I-gain  20000 N/m/s 
Integrator saturation  2000 N 
PI activation time  1.5 s 
PI error linear ramp 
up duration 
 0.5 s 
Feedforward    
Braking force Kff -18000 N 
Allocation    
Total longitudinal 
force 
Fx - N 
Allocation constant Kfl -2.04×10-4 Bar/N 
Allocation constant Kfr -5.10×10-5 Bar/N 
Allocation constant Krl -6.79×10-5 Bar/N 
Allocation constant Krr -1.70×10-5 Bar/N 
Brake pressure 
(front/rear left/right) 
Pfl/fr/rl/rr - Bar 
 
AVEC ’14 
 
 
Fig. 4 Position of tractor rear axle during unexpected 
brake intervention, starting at (0, 0) m. One black solid 
curve per driver, thick solid red is average of all drivers, 
dashed thick blue is reference run with fixed steering. 
Lines have been moved and rotated to get zero offset 
and heading at beginning. 
 
Continuing with the repeated runs, Fig. 6 and 7 give 
all trajectories and time series from the repeated runs. 
Four runs have been filtered out since the drivers 
pressed the accelerator pedal early on during the 
exposure. The average maximum lateral deviation 
observed is 0.13±0.03 m, again using 95% confidence 
level. There seems to be a reduction in lateral deviation, 
as drivers become aware of the true purpose of the test. 
To investigate this further a paired t-test was performed 
on the StW response. Fig. 8 show average change in 
StW angle from both the initial runs and the repeated 
runs. Also shown is the average difference between 
these runs per driver. Since each driver conducted only 
one initial run and two repeated runs the comparison is 
made between the initial run and the average of the 
repeated runs. In the four cases where runs are excluded 
only one repeated run is used. The difference between 
the lines indicate that there is a difference in reaction 
time between the runs. This is also clearly confirmed 
with the t-test which show a significant difference after 
0.5 s, run with 95% confidence level and 11 degrees of 
freedom. Looking at the time where the on average 
-5 deg is passed the difference is about 0.1 s. 
 
Fig. 5 Response to unexpected brake intervention, 
starting at time 0 s. Line styles same as in Fig. 4. 
 
 
Fig. 6 Position of tractor rear axle during repeated brake 
intervention. For line styles see Fig 4. 
AVEC ’14 
 
Fig. 7 Response to repeated brake intervention, starting 
at time 0 s. Line styles same as in Fig. 4. 
 
 
Fig. 8 Mean value of change in StW angle from the first 
initial runs and from the repeated runs. Also shown is 
the average difference between the initial run and the 
repeated runs. 
For the repeated runs done at 70 km/h the lateral 
deviation observed was at a similar level as in the 
repeated runs done at 50 km/h. The average maximum 
lateral deviation observed was 0.10±0.04 m, using 95% 
confidence level. The reaction time, before reaching a 
StW angle of -5 deg, was again lowered. 
 
4. CONCLUSION 
  
The combination of split-µ and automatic brake 
intervention has been tested in a truck with 12 unaware 
drivers. Even though drivers were unaware of brake 
intervention they were still all aware of being part of a 
study and consequently more observant than normal. 
The lateral deviation observed was higher in the first 
runs, when drivers were unaware, compared to repeated 
runs. An identified reason for this was shorter reaction 
time. Measured levels suggest that the risk of collision, 
due to lateral deviation, is low for an alert driver. For a 
distracted driver more support might be required. This 
was motivated by the runs where steering was held 
locked. As was obvious from the repeated runs a driver 
which knows what will come is more effective in 
reducing lateral deviation. This underlines the fact that 
the warning phase, which is already an important part of 
AEBS, should not be underestimated. 
Also beneficial would be a low value, or even 
negative value, of scrub radius since this limits the 
destabilising StW torque which has to be taken care of 
by the driver. Using even more sophisticated approaches 
for StW torque, like overlay torque guidance, might 
even improve the results further. This is however not 
obvious.  
Angle overlay, steer-by-wire or rear axle steer 
systems are other ways of reducing lateral deviation 
even further, e.g. see [4]. These also have the potential 
of reducing the stopping distance, when combined with 
brake controls. Stopping distance is obviously important 
when AEBS has activated due to risk of collision. 
No major difference in lateral deviation was 
observed when increasing speed from 50 km/h to 70 
km/h. Drivers did however show shorter reaction time 
in this case, most likely since it was the last part of the 
test. 
Finally said, the yaw rate frequency matched, in 
most runs, the resonance frequency of several truck 
combinations. It is however not clear whether drivers 
respond in the same way when having trailers connected. 
Therefore additional tests, including trailers running at a 
higher speed, are of importance to completely be able to 
investigate the combination of automatic braking on 
split friction. 
 
REFERENCES  
 
[1] Zhou, S.W., S.Q. Zhang, &, G.Y. Zhao, “Jackknife 
Control on Tractor Semi-Trailer during Emergency 
Braking”, Adv. Materials Research, Vol. 299-300, 
2011, pp. 1303-1306. 
AVEC ’14 
 
[2] Bareket, Z., D.F. Blower, &, C. MacAdam, 
“Blowout resistant tire study for commercial 
highway vehicles” The University of  Michigan, 
Technical report, 2000. 
[3] Pettersson, H.E., J. Aurell, & S. Nordmark, “Truck 
driver behaviour in critical situations and the impact 
of surprise”, DSC Europe, 2006. 
[4] Manning, W.J. &, D.A. Crolla, “A review of yaw 
rate and sideslip controllers for passenger vehicles”, 
Trans. of the Institute of Measurement and Control 
29.2, 2007, pp. 117–135. 
[5] ECE, “Addendum 12: Regulation No. 13”, Rev. 7, 
2011. 
[6] EU, “Commission Regulation (EU) No 347/2012”, 
2012. 
[7] Kharrazi, S., “Steering Based Lateral Performance 
Control of Long Heavy Vehicle Combinations”, 
Department of Applied Mechanics, Chalmers 
University of Technology, 2012. 
[8] ISO, “International standard 8855, Road vehicles 
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[9] Kiefer, R.J., LeBlanc, D.J. & Flannagan, 
“Developing an inverse time-to-collision crash alert 
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[10] Cheng, Q., “A study of truck driver deceleration 
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2012. 


Driver Response to Automatic Braking under Split Friction Conditions

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