“Braking Effectiveness Lessens at Higher Speeds”

Is This Still True Today??

By Daniel W. Vomhof III, EIT, ACTAR #484

Over the years, one of the “Urban Legends” within the field of accident reconstruction is

that as a vehicles speed increases, the deceleration rate from that speed will decrease ... in other

words, a drag factor from 30 mph will be higher than a drag factor from 60 mph given the same

vehicle on the same surface.

To test whether this “urban legend” holds true with today’s vehicles and today’s tires, a

review of the automotive magazines published between May 2006 and February 2007 was

from 80 mph-0.

In analyzing these tests, a deceleration factor for each test was calculated. This in turn

was used to calculate a theoretical, or “normalized”, braking distance from 60 mph-0 for all of

the tests. This was done in an attempt to allow the reader to more readily visualize the differences

for comparison purposes. Results for the normalized braking distances are displayed to two

decimal places for comparative purposes only, not because the author believe that braking

distances can be measured to the 100

th

of a foot outside of instrumented testing situations.

Within the reviewed tests, as shown in Figure 1, the deceleration rates ranged from 0.82-

1.16 G. The “average” deceleration from all three speeds was centered around 1.0 G. The sample

the 80 mph-0 and 100 mph-0 tests respectively.

Figure 2

Figure 3

Practical Implications

The practical implications of this data for the accident investigator as perceived by this

author are:

- Given modern tires and modern braking systems in vehicles which are properly maintained,

one should no longer expect a decrease in deceleration rate as the initial speed at which

braking is initiated increases.

- Premium/Luxury vehicles have a braking system which provides significantly better

The testing of these vehicles were all done on “test tracks”, but not all on the same track,

even within one publication. It is unknown what the coefficient of friction of the test track

surfaces are/were, nor how they compare to a “normal” roadway surface.

The magazines report the “best” numbers they can obtain .... its good PR. Further, the

magazines tune the vehicles before testing to assist in obtaining the best numbers possible. While

the summary test numbers do not specify the “type” of braking (locked wheel, incipient skid or

threshold braking, ABS, etc.), it is assumed by this author, based on reviewing hundreds of

magazine braking test reports, that these tests are all ABS braking tests. Regardless of whether

this assumption is true or not, the first sentence of this paragraph still applies.

No testing is commonly done by the automotive magazines at the lower speeds of

~30 mph-0. Therefore, based on the published data which has been reviewed, it is still possible,

Schmoe who drives back and forth to work every day.

Data

The individual test details, including publication and publication date, are attached at the

end of this article. In addition, this article and the spreadsheet in Excel format is available for

download from the Papers, Publications page of our website - www.4n6xprt.com - so that others

can study the data further.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Daniel W. Vomhof III is one of the principals at 4N6XPRT Systems, which publishes software

accident reconstructionist by ACTAR. He can be reached via e-mail at [email protected].

normalize - To make normal, especially to cause to conform to a standard or norm

minimum - Mathematics. The smallest number in a finite set of numbers.

average - number used to represent or characterize a group of numbers. The most common type

of average is the arithmetic mean. See median; mode.

maximum - Mathematics. The largest number in a set.

mode - In statistics, the most frequently appearing value in a set of numbers or data points. In the

any of the other figures. (See average; compare mean and median.)

are larger than the median as are smaller. In the group 4, 5, 6, 9, 13, 14, 19, the median is

numerals in the group, the median is usually defined as the number halfway between the

std deviation - In statistics, a measure of how much the data in a certain collection are scattered

around the mean. A low standard deviation means that the data are tightly clustered; a

high standard deviation means that they are widely scattered.

About sixty-eight percent of the data are within one standard deviation of the mean.

sample size - (statistics) The number of objects in the sample.

Quartile - A statistical term describing a division of observations into four defined intervals

data is ordered from smallest to largest with those observations falling below 25% of all

the data analyzed allocated within the 1st quartile, observations falling between 25.1%

and 50% and allocated in the 2nd quartile, then the observations falling between 51% and

75% allocated in the 3rd quartile, and finally the remaining observations allocated in the

4th quartile.

For some basic explanations of the statistical terms, with pictures, the reader is directed to

the following web site: http://www.robertniles.com/stats/