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Auto and Road User Journal
Copyright © 1997 by TranSafety, Inc.
September 3, 1997
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Study Shows That Motorists Drive at Reasonable Speeds

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.

TABLE 1
Compliance and Other Speed-Related Statistics

Speed Statistics urban-55 fringe-55 rural-65
average 59 mph 63 mph 74 mph
85th percentile speed 65 mph 69 mph 74 mph
percent exceeding speed limit 62% 89% 63%
percent >=10 mph over limit 15% 25% 9%

      1 mi = 1.6 km

The researchers concluded:

By and large, those results lend credence to the argument that motorists drive at speeds that they feel are appropriate, apparently independent of the posted speed. On urban-55 segments, where drivers are more confined by the geometric characteristics and more likely to encounter congestion, speeds are considerably slower than in fringe-55 areas, which are more open and less congested. This also indicates that freeway speed limits set closer to the 85th-percentile speed may be more appropriate and would lead to better compliance.

Interactions Between Traffic Characteristics and Accident Statistics

Since the test results were different for road sections with different speed limits and zones, the researchers tried controls to measure any differences in the crash data. They performed analyses of variance (ANOVAs) and found that the speed limit effect was insignificant, but the type of zone was significant, especially in daytime crashes.

Further analysis, described in detail in the report, also suggested that speed limits had relatively little effect on accident rates and frequencies. The type of zone, however, continued as a stronger contributor.

The researchers suggested a broader study, covering several years and using more highway types with more comprehensive traffic volume and speed tests.

Copyright © 1997 by TranSafety, Inc.


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