Research papers - Iowa's four- to three-lane analysis effortsEB/FB Comparison
|Safety Impacts of "Road Diets" in Iowa (2005 studies combined)|
A draft copy of a paper being prepared for consideration for publication in the ITE Journal (combines the results from the two 2005 studies above).
|The Effect of Four- to Three-Lane Conversion on the Number of Crashes and Crash Rates in Iowa Roads (2005)|
|Project report (PDF)
We analyze crash data collected by the Iowa DOT using Bayesian methods. The data set includes monthly crash numbers, estimated monthly traffic volumes, site length and other information collected at 30 paired sites in Iowa over more than 20 years during which an intervention experiment was set up.
|A summary submitted to the 2005 Mid-Continent Transportation Research Symposium, August 18-19, 2005, Ames, Iowa for an invited presentation on the topic:|
As part of a ISU statistics department creative component in cooperation with Iowa DOT's Office of Traffic and Safety, a full Bayes analysis of the reduction in crash frequency due to four-lane to three-lane conversions in Iowa was conducted. This study utilized monthly crash data and estimated volumes obtained from the Office of Transportation and Safety for 30 sites, 15 treatment and 15 control, over 23 years (1982-2004). The sites had volumes ranging from 2030 to 15350 weighted AADT during that timespan and were mostly located in smaller urbanized areas (the exception being one location in Des Moines). Each treatment site had different known intervention dates; therefore, the number of before and after crash records varied from site to site. Individual control sites were matched to each treatment site to provide a control sample similar to the treatment sample. In general, both treatment and control site crash history can be seen to experience a reduction. However, the reduction in treatment site crash frequency and rate after intervention are significantly more marked than that at the comparison sites. This differs from a previous Huang, et al. study whose data, even from a descriptive statistics standpoint, indicated very little reduction or difference between the two groups. Additionally, because monthly crash frequencies were used for analysis, it was possible to account for the seasonality effects on crashes, which should be expected given the seasonal weather patterns in Iowa. Given the random and rare nature of crash events, a hierarchical Poisson model where the log mean was expressed as a function of time period, seasonal effects, and a random effect corresponding to each site included was fitted to the crash frequencies. We adopted a full Bayesian (FB) approach for estimation of model parameters. Results indicate a 25 percent reduction in crash frequency per mile and a 19 percent reduction in crash rate. This differs again from the Huang, et al. study which reported a 6percent reduction in crash frequency per mile and an insignificant indication for crash rate effects. This difference is evident just by comparing the raw data from the two studies. The Iowa data, when graphed, indicates marked reductions whereas the Huang, et al., data indicate very little difference. Based on these Iowa FB results and results from a simple before/after analysis done as part of the causal study included in this paper, we are comfortable with the 25 percent and 19 percent reductions, especially as they fit practitioner expectations.
|As part of a ISU statistics department creative component in cooperation with Iowa DOT's Office of Traffic and Safety, a full Bayes analysis of the reduction in crash frequency due to four-lane to three-lane conversions in Iowa.|
|Before and After Study of Some Impacts of four- to 3-lane Roadway Conversions (2005)|
The focus of this research has been to evaluate the safety impacts of the conversion of 14 roadways from a four-lane undivided configuration to a three-lane configuration with one through lane in each direction and a two-way left-turn lane.
The safety impacts of the conversion of a four-lane undivided roadway to a 3-lane roadway are evaluated using classical before and after studies, of some 14 sites using yoked comparison pairs and a comparison to the cities in which the sites are located. Iowa DOT data bases were used to evaluate the changes in the frequency of crashes, rate of crashes, types of injuries, major causes of crashes, and key age groups. The results indicated that the frequency of crashes was reduced by about 24 percent, when other changes were accounted for, that serious injuries were reduced, that older drivers had a reduced risk of crashing, and that crash types related to left turns and stopped traffic were reduced.
|The safety impacts of the conversion of a four-lane undivided roadway to a three-lane roadway are evaluated using classical before and after studies, of some 14 sites using yoked comparison pairs and a comparison to the cities in which the sites are located.|
|Guidelines for the Conversion of Four-lane Undivided Roadways to three-lane Two-Way Left-Turn Lane Facilities (2001)|
|Project report (PDF)
Sponsored by the Iowa DOT's Office of Traffic and Safety of the Iowa Department of Transportation CTRE Management Project 99-54
|Project Web page||Related publications and presentations linked from project Web page|