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Antilock (ABS) Brakes
Safety Effects Underestimated?
ABS = Anti Blockier System (German abbreviation)
In spite of disagreements from other researchers, reports from (the industry independent) HLDI and NHTSA have resulted in a widespread and common belief ('authorized' even by
that accidents are not reduced with antilock brakes. A German University study 1983 on Munich taxi drivers is still referred to in this context (see
link to excerpt from
I (Lennart Strandberg)
commented on it in a paper to the 13th ESV Conference, see page 819 in the proceedings from
U.S. Department of Transportation
(No.DOT HS 807991, paper S7-O-08).
Objections from people in the automobile industry have been neglected, though the conclusions are biased.
Unfortunately, I have not heard of any attempt to clarify the influence from US differences in mileage. Therefore, I made a rough estimate with HLDI's own data from Report A-41 and Swedish mileage distribution between different year models.
Insurance study overlooks mileage?
HLDI does not take mileage into account when
are compared among the 1992 year models with standard ABS and the 1991 models without ABS. Though annual mileage tends to decrease with vehicle age, the HLDI unit of
(insured vehicle years) makes no difference between the younger ABS and older non-ABS cars. In the first HLDI report, twelve groups are compared (on an annual basis only), and in the Summary it is stated:
Then I found that the
ABS-cars exhibited less damage per mileage unit in all 12groups
distinguished by HLDI.
The ABS-favourable difference was
statistically significant in 11 out of 12comparisons.
You may find one of them in the diagram below.
Clicking on the image will enlarge and display it separately for downloading.
Evidence of mileage bias in data on the Web
Without any reservation or considerations of the mileage bias, HLDI and IIHS (Insurance Institute for Highway Safety) continue their devaluation of ABS with statements like
"Antilock Brakes Don't Reduce Fatal Crashes".
Like the past HLDI studies, this one may confirm the advantages of ABS, too. It is remarkable that IIHS does not present results adjusted for that the newer (ABS) cars are likely to be driven longer distances per year with less mileage on urban (low-speed) roads.
Such explanations may invalidate Risk Homeostasis speculations like "Drivers might feel overconfident and drive faster or take more risks" (quoted from
the body of IIHS news).
In this study, IIHS/HLDI's own data provide strong evidence of a mileage bias:
20 of 24 rows of their web page table,
the right-hand column exhibits greater values than the second column from the right. This is consistent with a greater mileage bias for the right-most column, where the average age difference between ABS and non-ABS cars is likely to be more pronounced.
The substantial column differences for pedestrians and bicyclists on the last rows are as expected, if the older cars are used more in urban areas.
The ABS cars seem to have a greater fatality rate in Single-Vehicle compared to Multiple-Vehicle crashes. This may also be an effect of confounding variables, such as urban mileage ratio.
However, steerability may have been put forward of stability in the first generations of ABS, resulting in
more violent yaw motions and side impacts than if front wheels lock up
(occupants are more susceptible to injuries in side- than in frontal impacts).
Such problems are taken into account in the ongoing development of GMA (Gier-Moment-Abwäschung) and modern ABS software.
NHTSA report on percentages vs. absolute numbers
The NHTSA report confuses me by putting forward some subgroup percentages that are unfavourable to ABS, though the total absolute number of crash cars is 1000 less with ABS (20000 cars) than expected from a random outcome (21000 cars). This is significant, indeed (Chi2-probability 3E-29, if my calculation is correct).
In spite of these data, clearly favourable to ABS, you may read on pagev in the NHTSA report:
(The referred Insurance Institute is part of the same organisation as is Highway Loss Data Institute, HLDI, mentioned above.)
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Misleading anti-looks at Antilocks
On November 15,
I finished two VETA papers in English on these observations:
we are not yet perfectly prepared for international distribution. However, we will try to send these reports with an invoice for postage and package (no more than 100 SEK), if you write down your name and address in an Email to
The following question is discussed in both papers listed above.
- Driving and crash safety with antilock brakes (ABS).
VETA Dialogue No.3, 6 pages, 2tables, 2figures (one
on the Web), 22 references.
- Misleading anti-looks at Antilocks.
Accident reductions with ABS brakes concealed?
VETA Prologue 951115, 3pp+2pp table, 2figs, 14refs.
Is ABS used for exercise by inexperienced safety researchers?
Perhaps the speculations on risk compensating behaviour are the main reason why people question the safety effect of ABS. At least, I have found certain investigators very eager to use ABS as an object for research exercise. It is quite easy to get data for statistical testing of hypotheses in this context, since ABS is so common without being excluding.
When the outcome of a statistical test does not show any significant differences, medical researchers suggest that the results are negative. However, it has not of course been proved that ABS brakes or other measures are without any effect, only that nobody has succeeded in proving any effect. This is a vital scientific principle which should apply in the area of traffic safety as it does within epidemiology.
It may be reasonable to assume that there is no real effect, if
many independent studies show negative results.
In the ABS issue, though, literature surveys and references often can be tracked down to one single study: the Munich taxi experiment in the early 1980s.
This driver behaviour and attitude towards ABS may be representative for the early 1980īs. Unfortunately, another part of the Munich study is also referred to in many contexts as proof that ABS brakes do not improve traffic safety. In retrospective accident analyses the results were negative: no significant differences were found between ABS and non-ABS taxi cabs. Therefore, it was concluded that ABS had no effect.
A closer inspection, however, reveals that there are many confounders in the first analysis (1981-1983) based on 21 cars equipped with ABS brakes. With so small numbers it is remarkable that these negative results are considered a proof of ABS inefficiency more than a decade later.
In the second Munich evaluation period (1985/86) there were even fewer cars: 10 cars without and 10 cars with ABS brakes. Based on these circumstances it seems fairly natural that the risk indexes for all accident types did not differ with any degree of statistical significance.
If I am not mistaken above, ABS has been devaluated through overinterpretations of immature research. However, since the HLDI and NHTSA studies have been reported in detail, it is possible for the scientific community to check the methods. If we don't, nobody should blame the misunderstandings on NHTSA or on HLDI with their well-defined evaluation of car crashworthiness.
Without such clear definitions, conclusions could not be questioned so easily. It is also thanks to the NHTSA open-mindedness and to the
that it is possible to discover and constructively criticise research, as in all scientific work. I can recommend a visit to the NHTSA
report download service.
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Antilock (ABS) Brakes: How to use them?
Irrespective of accident statistics,
support the warnings (on inadequate driver reactions) given by
and by NHTSA, also explaining ABS-technology in the downlodable
PDF-file dated 03/21/95 (92K).
I strongly recommend ABS-car drivers to study the
from one of the auto-manufacturers. And don't miss Autonet's comprehensive guide with technical details and pictures of
ABS and its components
ABS-questions and answers.
An excellent overview of ABS-related Tech & Trends from a driver's perspective was given by Bill Visnic on page 101 in the December -95 issue of
WARD'S Auto World.
Whether ABS or not,
brakes must be properly maintained
to avoid failures and
spin-outs from rear wheel lock-up
which is likely to occur if you try to stop the car by braking the rear wheels only (for instance with the parking brake or by downshifting in a rear wheel driven car).
If you excuse my SwEnglish, I would also like to contribute a translation from a recent Swedish press-release on the
VETA study of braking capability on slippery surfaces in
experiments with 66 ordinary drivers.
- When the brake pedal starts vibrating, you should kick it down even harder in order to brake all wheels to their peak friction force.
You may namely perceive the ABS-pulses though only one wheel tends to lock-up.
Then, the other wheels may roll on less slippery surfaces with a lot of friction force left to utilise.
- At the same time you may steer the car away from the most slippery track.
- Don't turn the steering wheel like a pendulum, though!
Utilising steerability may cause instability and spin-out.
- Remember that ABS may increase the braking distance on certain surfaces, where a locked wheel 'digs itself down' to greater adhesion.
- Many people are unaware of the wheel slip error in the ABS computer, introduced by different rolling radii,
when a space saver spare wheel is mounted
New website and 2-language Homepage
Tvåspråkig hemsida och ny domän
are hosting webpages ...
To be conscious that you are ignorant is a great step to knowledge
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Who is the guy writing by I, just nearby?
Brief curriculum vitae for Lennart Strandberg,
founding chairman of
and its temporary webmaster.
Born in the Dalecarlia province of
200kilometres south of Stockholm (capital city of Sweden) together with a wonderful
Victor (born 1983) has opened a site by himself on
like Need for Speed, Descent, Slipstream 5000, Doom, Dark Forces etc.
Practical experience as a test and
racing driver (more the
European Rally or BTCC style than the
American Indy Car or NASCAR style, though).
My driver's licence covers all road vehicles (A BE CE DE in Sweden).
Occupation: Traffic safety research consultant.
Charter chairman of the
VETA research society.
Professor in accident research at
NIWL (presently off-duty).
Until December 1994: driving and traffic safety research at the Swedish Road and Transport Research Institute,
Member of Linköping-Filbyter
(district: 2410, classification: Driving safety research).
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