Safety Impacts of "Road Diets" in Iowa

Institute of Transportation Engineers. ITE Journal, Dec 2006 by Stout, Thomas B, Pawlovich, Michael D, Souleyrette, Reginald R, Carriquiry, Alicia

Table 2 presents the change in crash frequency determined from the B/A analysis for the conversion sites, their cities and the yoked or comparison sites. For the study sites alone, the crash frequency after the conversions had an average reduction of 50 percent. After subtracting the change in citywide crashes, the net effect of the conversion is a reduction of about 24 percent. When the reduction of the comparison sites is subtracted from the study sites' average reduction, the net effect would be a reduction of about 39 percent.

Iowa DOT was equally interested in changes in crash types. Table 3 shows the change in crash frequency from the B/A analysis in types of crashes. One can note a significant reduction in those types of crashes most often related to turning vehicles.

Iowa DOT was particularly interested in the potential decrease in injury crash frequency due to road diets. Figure 2 graphically summarizes the results of the B/A analysis of injury crashes. When compared to the city crashes, major injury crashes at the converted sites were reduced by 11 percent, minor injury crashes by 30 percent and possible injury crashes by 31 percent. Overall, injury crashes were reduced by 34 percent.

Due to their rarity, fatal crashes were included as major injury crashes. Major injuries are those that are serious or incapacitating, such as broken bones and internal injuries. Minor injuries are non-incapacitating, such as bruises or scrapes. Possible injuries are those where the victim complains of pain but has no other signs.

Iowa has a relatively high proportion of elderly persons. The 2004 U.S. Census estimates that nearly 15 percent of the population is elderly, versus less than 13 percent for the United States as a whole. Iowa DOT requested an investigation into whether older drivers benefit more from the conversions.

Figure 3 shows the breakdown by driver age for crashes at the study segments for the before and after cases. The result of these conversions is to reduce the proportional involvement of older drivers as well as that of drivers under 25.

CONCLUSIONS

A number of benefits can be realized from the conversion of urban four-lane undivided roadways to three-lane cross sections in selected locations where physical or environmental constraints prohibit options that involve widening. The benefits found in this study include the following:

* A 25-percent reduction in crash frequency per mile and a 19-percent reduction in crash rate. This differs from the Huang, Stewart and Zegeer study, which reported a 6-percent reduction in crash frequency per mile.

* A 34-percent reduction in the number of injury crashes as well as a reduction in the severity of the crashes that do occur.

* Reductions in the involvement of age groups that are traditionally at risk, those 25 and younger as well as those 65 and older.

* A significant reduction in the number of crash types related to left turns and stopped traffic.

References

1. Huang, H.F., J.R. Stewart and C.V. Zegeer. "Evaluation of Lane Reduction 'Road Diet' Measures on Crashes and Injuries." Transportation Research Record, No. 1784 (2002): 80-90.