This study tabulated and analyzed crash rates for eastbound and westbound segments of I-94 in the vicinity of County Stadium (since demolished) near Milwaukee, Wisconsin. An electronic billboard began operation on April 13, 1984. Crash rate data was collected for approximately three years prior to sign operation (from 1/1/81) until three years after operation began (12/31/87). Effects were broken down by type of crash (side-swipe, rear-end). Data were analyzed for the one year after the sign became
operational, to analyze any novelty effect, as well as for the three year periods before and after the sign became operational. Crash rate was calculated as number of crashes per million vehicle miles of travel (VMT).
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The sign is described as a variable message sign that changed images on average 12 frames per minute. This suggests that each image was displayed on the sign for five seconds. No information is provided as to the sign’s display technology, brightness, or method of change. It is not known, for example, whether message changes occurred instantly, or whether some visual special effects, such as wipe, dissolve, etc., were employed. Neither the size of the sign nor its height above grade is specified. The sign is obviously two-sided since it is visible to both eastbound and westbound traffic. It is located adjacent to the westbound traffic lanes.
The study used the crash rate in the three years prior to the sign’s operational date as the baseline. Findings showed that for eastbound traffic, total crashes increased by 43% in the first year, and 36% over the three year post-operational period when compared to the baseline condition. In the same periods, side-swipe crashes increased 80% and 8%, and rear-end crashes increased 60% and 21%. For westbound traffic, total crashes decreased by 12% in the first year, but increased by 21% over the three year post-operational period. Sideswipe crashes increased 123% in the first year, and 35% over the three year interval, whereas rear-end crashes decreased 29% in the first year, and then increased by 35% over three years.
The author posits two reasons why westbound crashes were generally lower than those for eastbound motorists. First he describes a merge area for westbound drivers caused by northbound and southbound traffic on US-41 merging onto westbound I-94, and states that the roadway configuration causes this traffic to slow as it enters the area, thus reducing congestion through what he describes as “metering.” Second, the author indicates that the sign was more readable to eastbound than to westbound traffic.
The author concludes that “it is obvious that the variable message sign has had an effect on traffic, most notably in the increase of the side-swipe rate,” and suggests that “it may be beneficial to introduce traffic responsible variable message signs into the area. Signs could function at rates proportional to traffic flow and density in the viewing area.” This study has the strengths of a typical crash rate analysis. Although it cannot address questions of crash causation, the study can be used to determine that there were
correlations between the operation of the advertising sign and the increase in crash rates in areas where the sign was visible.
Apparently five types of crashes were coded from the accident reports: rear-end,
sideswipe, fixed object, other, and unknown. The report reviews only the data for the first two crash types, and this is appropriate. Both side-swipe and rear-end crashes are
indicative of driver inattention or distraction, although this roadway section includes a complex interchange where merges and lane changes are likely. Poor signage and markings, difficult geometry, lane drops and other roadway characteristics could have been present (these roadway and traffic characteristics are not described) which might suggest elevated crash rates of these types.
When the goal is to determine whether a particular object or feature (in this case an electronic changeable message sign) caused crashes to occur, or caused the overall crash rate to increase, a study that is limited to an analysis of crash rates cannot answer this question. This is because the study is limited to post-hoc statistical tabulations. The study does not address, and clearly did not control for, the possibility that other changes took place in the roadway section studied in addition to the operation of the billboard. For example, changes to speed limits, police enforcement activities, reporting methods, use patterns, construction, development adjacent to the roadway, and many other factors, might have been present, and might have contributed to changes in crash rates. There was apparently no attempt made to identify whether any such factors may have occurred during the study period. In addition, the study apparently did not utilize a control section of roadway that might have overcome some of these potential weaknesses. Had the authors chosen a similar section of I-94 in the same general vicinity as the study section, but in which no advertising sign was introduced, they might have been able to compare before-and-after crash rates for the same period, but without the presence of the sign. This would have strengthened their ability to demonstrate that it was the presence of the sign, rather than some other factor, that related to the elevated crash rates.
The author states that the study areas included “all places where the variable message sign can be viewed by a motorist….” Since the precise billboard location is not identified on the site maps included with the report, it is not possible to determine whether all crashes occurred at locations where drivers would have had a clear view of the billboard prior to the crash.
Although the study evaluated crash rates before and after the introduction of an electronic variable message billboard with a message change interval of approximately every five seconds, no additional information is provided to enable the reviewer to determine the type of sign, the display technology, or the operational characteristics. As stated above, although crash rate data can supply valuable information relative to overall traffic safety in an area, it is not possible to identify a cause and effect relationship without far greater control of other, possibly relevant, variables – something that is quite difficult to do in a real world environment and with a post-hoc analysis of police accident reports.