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The Federal Study that Finds Speed Limits Irrelevant...
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Subject: The Federal Study that Finds Speed Limits Irrelevant...
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From: kevinatk@home.com (Kevin Atkinson)
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Date: 24 Jul 1995 19:26:13 GMT
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Keywords: federal-study, study, speed-limits, speed, limits
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Newsgroups: rec.autos.driving, dc.driving, ca.driving
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Organization: Clark Internet Services, Inc., Ellicott City, MD USA
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Xref: news.clark.net rec.autos.driving:77234 dc.driving:3319 ca.driving:18368
Here is part of the federal study that NMA dug up after the goverment
tried to bury it. The federal government spent over half a million
dollars and five years on it; it is not just a trivial or academic study.
This document and a lot more can be found at my Speed Limit home page at
http://www.clark.net/pub/kevina/sl/
EFFECTS OF RAISING AND LOWERING SPEED LIMITS
Final Report
(Abstract and Finding)
Report No. FHWA-RD-92-084 October 1992
U.S. Department of Transportation Research, Development, and Technology
Federal Highway Administration Turner-Fairbank Highway Research Center
6300 Georgetown Pike
McLean, Virginia 22101-2296
The objectives of this research was to determine the effects of
raising and lowering posted speed limits on driver behavior and
accidents for non-limited access rural and urban highways. Speed and
accident data were collected in 22 States at 100 sites before and
after speed limits were altered. Before and after data were also
collected simultaneously at comparison sites where speed limits were
not changed to control for the time trends. Repeated measurements were
made at 14 sites to examine short - and long-term effects of speed
limit changes.
The results of the study indicated that lowering posted speed limits
by as much as 20 mi/h (32 km/h), or raising speed limits by as much as
15 mi/h (24 km/h) had little effect on motorist' speed. The majority
of motorist did not drive 5 mi/h (8 km/h) above the posted speed
limits when speed limits were raised, nor did they reduce their speed
by 5 or 10 mi/h (8 or 16 km/h) when speed limits are lowered. Data
collected at the study sites indicated that the majority of speed
limits are posed below the average speed of traffic. Lowering speed
limits below the 50th percentile does not reduce accidents, but does
significantly increase driver violations of the speed limit.
Conversely, raising the posted speed limits did not increase speeds or
accidents.
_________________________________________________________________
Summary of Findings
The pertinent findings of this study, conducted to examine the effects
of lowing and raising posted speed limits on nonlimited access rural
and urban highways, are listed below:
* Based on the free-flow speed data collected for a 24-h period at
the experimental and comparison sites in 22 States, posted speed
limits were set, on the average, at the 45th percentile speed or
below the average speed of traffic
* Speed limits were posted, on average, between 5 and 16 mi/h (8 and
26 km/h) below the 85th percentile speed.
* Lowering speed limits by 5, 10, 15, or 20 mi/h (8, 16, 24, or 26
km/h) at the study sites had a minor effect on vehicle speeds.
Posting lower speed limits does not decrease motorist's speeds.
* Raising speed limits by 5, 10, or 15 mi/h (8, 16, or 25 km/h) at
the rural and urban sites had a minor effect on vehicle speeds. In
other words, an increase in the posted speed limit did not create
a corresponding increase in vehicle speeds.
* The average change in any of the percentile speeds at the
experimental sites was less than 1.5 mi/h (2.4 m/h), regardless of
whether the speed limit was raised or lowered.
* Where speed limits were lowered, an examination of speed
distribution indicated the slowest drivers (1st percentile)
increased their speed approximately 1 mi/h (1/6 km/h). There were
no changes on the high-speed drivers (99th percentile)
* At sites where speed limits were raised, there was an increase of
less than 1.5 mi/h (2.4 km/h) for drivers traveling at and below
the 75th percentile speed. When the posted limits were raised by
10 and 15 mi/h (16 and 24 km/h), there was a small decrease in the
99th percentile speed.
* Raising speed limits in the region of the 85th percentile speed
has an extremely beneficial effect on drivers complying with the
posted speed limits.
* Lowering speed limits in the 33rd percentile speed (the average
percentile that speed were posted in this study) provides a
noncompliance rate of approximately 67 percent.
* After speed limits were altered at the experimental sites, less
than one-half of the drivers complied with the new posed limits.
* Only minor changes in vehicles following as headways less than 2s
were found at the experimental sites.
* Accidents at the 58 experimental sites where speed limits were
lowered increased by 5.4 percent. The level of confidence of this
estimate is 44 percent. The 95 percent confidence limits for this
estimate ranges from a reduction in accidents of 11 percent to an
increase of 26 percent.
* Accidents at the 41 experimental sites where speed limits were
raised decreased by 6.7 percent. The level of confidence of this
estimate in 59 percent. The 95 percent confidence limits for this
estimate ranges from a reduction in accidents of 21 percent to an
increase of 10 percent.
* Lowering speed limits more than 5 mi/h (8 km/h) below the 85th
percentile speed of traffic did not reduce accidents.
* The indirect effects of speed limit changes on a sample of
contiguous and adjacent roadways was found to be very small and
insignificant.
_________________________________________________________________
Conclusion
The primary conclusion of this research is that the majority of
motorist on the nonlimited access rural and urban highways examined in
this study did not decrease or increase their speed as a result of
either lowering or raising the posted speed limit by 4, 10, or 15 mi/h
(8, 16, or 24 km/h). In other words, this nationwide study confirms
the results of numerous other observational studies which found that
the majority or motorist do not alter their speed to conform to speed
limits they perceive as unreasonable for prevailing conditions.
The data clearly show that lowering posted speed limits did not reduce
vehicle speeds or accidents. Also, lowering speed limits well below
the 86th percentile speed did not increase speeds and accidents.
Conversely, raising the posted speed limits did not increase speeds
and accidents. The majority of motorist did not drive 5 to 10 mi/h (8
to 16 km/h) above the posted speed limit when speed limits were
raised, nor did they reduce their speed by 5 or 10 mi/h (8 to 16 km/h)
when speed limits were lowered.
Because there were few changes in the speed distribution, it is not
surprising that the overall effects of speed limit changes on
accidents were minor. It is interesting to note that compliance
decreased when speed limits were lowered and accidents tended to
increase. Conversely, when compliance improved after speed limits are
raised, accidents tended to decrease.
Based on the sites examined in 22 States, it is apparent that the
majority of highway agencies set speed limits below the average speed
of traffic as opposed to setting limits in the upper region of the
minimum accident risk band or about 85th percentile speed. This
practice means that more than one-half of the motorist are in
technical violation of the speed limits laws.
Although there are variations from State to State, on average, speed
limits were posed 5 and 16 mi/h (8 and 26 km/h) below the 85th
percentile speed. As all States use the 85th percentile as a major
criterion for establishing safe and reasonable speed limits, it is
surprising that the new speed limits posted on the experimental
sections examined in this study deviated so far from the 85th
percentile speed. There are several plausible reasons. Once commonly
cited reason for posting unreasonably low speed limits is public and
political pressure. While individuals and politicians clearly
influence some speed limit decision, there are other factors involved.
Although the 85th percentile speed is used as the major guideline in
setting speed limits, other factors such as land use, pedestrian
activity, accident history, etc., are often subjectively considered in
the decision making process. Together, these factors can account for
sped limits that are set 10 mi/h (16 km/h) below the 85th percentile
speed. In addition, the 85th percentile speed is often estimated based
on a minimum of 200 vehicles or 2 h sample. This process does not take
into account the wide hourly fluctuations in the 85th percentile speed
over a 24-h period. Furthermore, the vehicle selection process use of
radar which is detected by motorist contribute to a bias sample, i.e.,
usually lower then the average 24-h 85th percentile speed.
Although the study sites could not be randomly selected, they
represent a wide range of rural and urban conditions, traffic volume,
and regional situations. As large changes in the posted speed limit
did not create a meaningful increase or decrease in the motorists'
speeds at the study sites, it is plausible that this effect would also
be found on other nonlimited rural and urban access highways.
The data collected during this study indicate that there are no
benefits, either from a safety or operational point of view, from
establishing speed limits less than the 85th percentile speed. This
does not mean that all speed limits should be raised. Traffic and
engineer investigations should be conducted to obtain an accurate
measure of the speed distribution. Greater emphasis should be placed
on using the 85th percentile speed in setting safe and reasonable
speed limits. These studies should be repeated as land use and traffic
characteristics change.
The information provided in this report will be useful to highway
agencies, enforcement officials, and other involved in establishing
uniform safe and reasonable speed limits on the nation's highways. The
graphics, such as figure 10 on p.15 [sorry don't have], can be used to
illustrate the effects of speed limit changes on vehicle speeds. As
shown below, figure 41 [see figure41.gif] (which shows the changes in
accidents, as well as the 95th percentile confidence limits of the
changes) can be used to illustrate the effects of lowering and
raising speed limits in accidents. This figure should only be used
by persons who have read the accident analysis section in this
report and have a basic understanding of the analysis results.
_________________________________________________________________
A print copy of this info can be obtained from the National Motorist
Association. They can be contacted at 608/849-6000;
nma@genie.geis.com; or 6678 Pertzborn Road, Dane, Wisconsin 53529.
--
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