A study of speed limits on different freeway types in Michigan failed to show that speed
was a substantial contributor to more frequent or more serious crashes. It showed that
compliance with speed limits was not necessarily an accurate measure of safety.
Although more crashes occur in urban areas, as can be expected from congestion and
the need to react to other vehicles, drivers seem to choose speeds similar to the design
speeds for different types of roads. The research suggests that lowering speed limits
arbitrarily does not affect traffic safety. Speed limits and speed zones would be more
effective if they were based on geometrics, traffic characteristics, and safety benefits
rather than popular conceptions.
Matthew Thornton and Richard W. Lyles reported their research on this topic in
"Freeway Speed Zones: Safety and Compliance Issues." The report appeared in the
Transportation Research Board's Transportation Research Record No. 1560, Traffic
and Highway Safety: Occupant Restraints, Safety Management, and Emergency and
Commercial Vehicles, published in 1996.
BACKGROUND
In 1987 Congress allowed increased speed limits, from 55 to 65 miles per hour (mph),
on rural Interstates and some U.S. routes. Population density was the criterion for
speed zone boundaries. Since many urban freeways had rural elements such as wide
medians and widely separated interchanges, transportation officials and engineers
wanted more speed zone changes. In 1990 the U.S. Department of Transportation
allowed the states to be more flexible in setting 65-mph limits.
In many urban areas, speed limits change from 65 to 55 mph even when no differences
in road characteristics demand such a change. The speed limit decreases simply
because the roadway is within an urban area. For speed zones to be appropriate,
accepted engineering practice suggests that factors in addition to population density
should be considered.
CRITERIA FOR ESTABLISHING SPEED ZONES AND LIMITS
Some important criteria to consider when setting speed limits are traffic volume,
geometry, compliance, and safety.
Traffic Volume
At some point, the volume of traffic affects its flow and forces motorists to slow. This
factor is important because:
- Traffic volume is often used as a reason to lower speed limits.
- Peak volume occurs at certain times and for certain durations.
- Merging volumes, densities of traffic, and increased speeds all affect drivers'
abilities to merge and change lanes.
- Each highway section has a certain carrying capacity.
Geometric Design
The researchers emphasized geometry in this study, because it is often constant
across urban boundaries. To set speed limits, they found that these factors should be
considered:
- lane, shoulder, and median widths;
- design speed; and
- whether the highway section is depressed or elevated.
Speed Compliance and Driver Behavior
The generally accepted determination of speed limits is the 85th-percentile speed--the
speed that 85 percent of motorists do not exceed. Supporters assert that this speed
"accommodates the safe and prudent driver" and that changing a posted speed limit
makes little difference in the 85th-percentile speed. Studies show that raising speed
limits to the 85th-percentile does not increase crashes. Speed limits at this level are
supported because they are realistic. Realistic speed limits decrease violations and
speed variation, and aid traffic flow.
The 85th-percentile speed limit guideline is not universally accepted. Some highway
safety experts maintain that 65 mph "is simply not as safe" as 55. Freeway speed limits
are often not determined by 85th-percentile speeds or by geometry, volume, or driver
behavior. Since urban boundaries create these speed limits, it appears that some
freeway sections have unrealistic speed limits.
Safety Issues
Political opinion maintains that safety increases with restrained speed. Many people
assume that the rural highway speed limit reduction to 55 mph in 1974 increased
highway safety, and the increase to 65 mph in 1987 reversed this effect.
Although crashes and related fatalities declined after the Interstate speed limit changed
to 55 mph, this decrease both continued an existing trend and coincided with
decreases on freeways with unchanged speed limits. Improved medical services and
better vehicle and road design helped increase roadway safety. And, even though the
speed limit remained at 55 mph for 13 years, drivers did not widely comply.
Many states conducted accident studies when speed limits increased to 65 mph.
Comparisons of accident rates produced varied results.
The researchers point out a lack of validity in the argument that the lower speed limit
reduced crashes, if motorists were not driving at the lower speed. They conclude that
speed is one factor among many that affect highway safety.
CASE STUDY: FREEWAY SPEED ZONES IN MICHIGAN
The researchers focused on selected freeway sections in Michigan to examine accident
rates and driver compliance in speed zones. They tested the hypothesis that freeway
sections in "urban fringe areas" were most likely to be zoned inappropriately. A fringe
area is within an urban area, has a speed limit of 55 mph, and has rural design
characteristics. These areas include:
- wide medians;
- wide lanes and shoulders;
- widely spaced access ramps;
- few weave areas; and
- at-grade design.
In contrast, "regular urban segments" had a speed limit of 55 mph with narrow medians,
closely spaced ramps, extended weave areas, and elevated or depressed sections.
Freeway segments with speed limits of 65 mph were termed rural.
The study included three zone types: urban-55, fringe-55, and rural-65. Most locations
had all three segment types in proximity to each other.
Study Locations
The research sites incorporated both interstates and local freeway locations in and
near several major urban areas. Of the 54 segments studied, 27 were rural, totaling
225 miles; 17 were fringe, totaling 64 miles; and 10 were urban, totaling 42 miles.
Data Collection, Stratification, and Analysis Framework
The Michigan Department of Transportation (MDOT) supplied the accident data from its
files for 1991, the volume and speed compliance data from observations during the
summer of 1993, and the geometric data from site visits, maps, and drawings. MDOT
considered the variance in years insignificant, since there was no notable change in
enforcement or accident and speed data during that time. The researchers
acknowledged that it would be better to have speed and crash data for a longer time
period and suggested that their results showed a need for more extensive research.
The research separated day/dry, day/wet, night/dry, and night/wet conditions for
analysis. Evaluation of this data found similar results under these various conditions.
Fatal crashes occurred rarely.
Differences in Accident Frequencies, Distributions, and Rates
Data collected during the project showed accident types varied among the different
sites.
Rear-end, fixed-object, and overturn crashes happened more often in the urban-55
areas. Animal-related crashes and property-damage-only (PDO) crashes occurred
more often in rural-65 areas. Fatal crashes showed the same trend as the PDOs. The
researchers recognized that sample sizes were small and road lengths were unequal.
Even so, they remarked that the property damage and injury distributions were the
opposite of the generally supposed effect of higher speed on accident data.
The data showed that all types of crashes occurred more frequently in the urban-55
areas. Although the urban rates "are somewhat inconsistent with other urban and rural
accident rates for Michigan," the ratio of the rates is similar--with urban rates more than
twice as high as rural rates.
Differences in Speed Compliance and Related Statistics
The researchers also looked at compliance with speed limits and other speed-related
variables. They found that the 85th-percentile speed for the urban-55 sections was 65
mph. For the rural-65 sections the 85th-percentile speed was 74 mph. The fringe-55
areas "serve as transitions between the urban and rural segments with in-between
speed statistics."
Table 1 below shows that the percent of drivers exceeding speed limits was high, with
the "worst case" in the fringe-55 areas where 25% of the vehicles were at least 10 mph
over the speed limit.
