An earlier post of mine described how appropriate speed limits are set and why it is important to reduce speed variation within traffic, but I did not effectively present the argument as to why this is important.

An observational study conducted by AAA Foundation for Traffic Safety over the course of 4 years. The study chose 36 locations to conduct their study. These locations included rural interstates, urban interstates, and
fre
eways/expressways. The researchers chose sites that are similar to each other, to limit confounding variables, and that had adequate accident data, adequate exposure data, and that were easy to collect new data. Since the study was conducted  over the course of four years, it is reasonable to assume that the sample size is large enough. This study observed a direct correlation between speed variance and accident rate. However, one of my concerns with this study is that it uses past accident data from the Virginia Department of Transportation and present, collected data with regard to drivers speed. Therefore, the accidents rates used in the study are of events in the past, and the speed data was collected at a later time. There could be a difference of speed variance over that time. Also, the location of sites is limited to only Virginian roads. Much of the country, such as the mid west and west, has a different topography that may affect drivers visibility and road patterns, which may affect speed. Also, there is a cultural driving difference in different regions. For example, New England drivers are notorious for their fast and aggressive driving. It would like to see similar studies in different regions.

The AAA study concluded that speed variance would best be minimized when speed limits were set 5-10 mph below the design speed of the roadway. Their conclusion is backed by the mathematical models they developed from their data. This conclusion seems idealistic because it was substantiated on mathematical models. Usually, what may seem practical and logical on paper does not play out in reality, especially when humans are involved. The study does a good job observing the relationship between speed variance and accident rate. However, I wanted to see other studies that focused on the best way to reduce speed variation.

Another study conducted by the Michigan State Police addresses speed limits and speed variance in a more realistic way. They conducted a test on the same highway that had speed zones of both 55mph and 70mph. Their study found that in the 55 mph zones, the average speed was 66.4 mph and a variance of 36.1 mph. On the contrary, in the 70 mph zone had an average speed was 67.7 mph and variance of 27.8 mph, a 33% decrease. Also, outliers also decreased in this study. In the 55 mph zone 2.1% of drivers exceeded 80 mph while only 1.1% exceeded 80 mph in the 70 mph zone. This study though was quite short and it was done for different amounts of time in each speed zone. In the 55 mph zone, data was only collected for 17 minutes, but in the 70 mph zone, the data was only collected 18 minutes. Therefore, only 658 cars’ speeds were recorded in the 55 mph zone compared to 721 cars in the 70 mph zone. The study did not also specify the time of day that the it was conducted. Although the study sheds light on the matter and limiting speed variance by setting speed limits at the 85th percentile, there are flaws with the observational study. More studies should be conducted for a longer period of time, in several different states, and at similar time of day to determine the best speed limiting speed variance.

This study also refutes opponents arguments against adjusting speed limits to the 85th percentile. The Insurance Institute for Highway Safety (IIHS) website state, “The 85th percentile is not a stationary point. It is, rather, a moving target that increases when speed limits are raised. If speed limits are raised to meet a current 85th percentile speed, a new, higher 85th percentile speed will likely result.” Average speed increasing with the speed limit as hypothesized by this statement is not backed by any scientific studies.  The study discussed earlier conducted by the Michigan State Police is evidence that rejects this alternative hypothesis. The null hypothesis, raising speed limits will not raise average speed, is either correct or a false positive according to the Michigan study. Like I mentioned before, the difference in average speed in the 70 mph zone was only 1.3 mph faster than in the 55 mph zone.

The IIHS also states on there website to combat raising speed limits to the 85th percentile by stating how about 10,000 automotive deaths were due to speed-related incidents.  They conveniently kept that statistic ambiguous to make the consumer of their website think that statistic only accounts for speeding above the posted limit. What the IIHS failed to mention about the National Highway Traffic Safety Administrations (NHTSA) statistics that they cited is the numerous confounding variables contribute to the the “speed-related” fatalities. The confounding variables are included in the very definition of a speed-related crash, which is defined as, “any crash in which the police indicate that one or more drivers involved was exceeding the posted speed limit, driving too fast for conditions, driving at a speed greater than reasonable or prudent, exceeding a special speed limit or zone, or racing.” It also states a disclaimer that , “there is a significant overlap between alcohol involvement and speed. Many speed-related crashes involved alcohol and vice-versa.” This was purposefully not mentioned on the IIHS website because it would weaken their argument. For example, a 2007 NHTSA report stated, “In 2007, 40 percent of drivers with a BAC of .08 g/dL or higher involved in fatal crashes were speeding, compared with only 15 percent of drivers with a BACof.00g/ dL involved in fatal crashes.” Although this is a statistic from 2007, 2013 should not differ by much. Therefore, it is common for a speed-related crash to also be an alcohol-related crash.

After delving into the studies and statistics even deeper than before, I am convinced that speed variance is more of a threat to road safety than speeding. I am lead to believe by the Michigan State Police that the most appropriate speed limit should be the 85th percentile, but I would like to see more extensive studies with similar conclusions.