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, can be used to illustrate the effects of speed limit changes on vehicle speeds. As shown below, figure 41 (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.
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