The Effect of Four-Lane to Three-Lane Conversion on the Number of Crashes and Crash Rates in Iowa Roads, June 2005

We analyze crash data collected by the Iowa Department of Transportation 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. The intervention consisted in transforming 15 undivided road segments from four-lane to three lanes, while an additional 15 segments, thought to be comparable in terms of traffic safety-related characteristics were not converted. The main objective of this work is to find out whether the intervention reduces the number of crashes and the crash rates at the treated sites. We fitted a hierarchical Poisson regression model with a change-point to the number of monthly crashes per mile at each of the sites. Explanatory variables in the model included estimated monthly traffic volume, time, an indicator for intervention reflecting whether the site was a “treatment” or a “control” site, and various interactions. We accounted for seasonal effects in the number of crashes at a site by including smooth trigonometric functions with three different periods to reflect the four seasons of the year. A change-point at the month and year in which the intervention was completed for treated sites was also included. The number of crashes at a site can be thought to follow a Poisson distribution. To estimate the association between crashes and the explanatory variables, we used a log link function and added a random effect to account for overdispersion and for autocorrelation among observations obtained at the same site. We used proper but non-informative priors for all parameters in the model, and carried out all calculations using Markov chain Monte Carlo methods implemented in WinBUGS. We evaluated the effect of the four to three-lane conversion by comparing the expected number of crashes per year per mile during the years preceding the conversion and following the conversion for treatment and control sites. We estimated this difference using the observed traffic volumes at each site and also on a per 100,000,000 vehicles. We also conducted a prospective analysis to forecast the expected number of crashes per mile at each site in the study one year, three years and five years following the four to three-lane conversion. Posterior predictive distributions of the number of crashes, the crash rate and the percent reduction in crashes per mile were obtained for each site for the months of January and June one, three and five years after completion of the intervention. The model appears to fit the data well. We found that in most sites, the intervention was effective and reduced the number of crashes. Overall, and for the observed traffic volumes, the reduction in the expected number of crashes per year and mile at converted sites was 32.3% (31.4% to 33.5% with 95% probability) while at the control sites, the reduction was estimated to be 7.1% (5.7% to 8.2% with 95% probability). When the reduction in the expected number of crashes per year, mile and 100,000,000 AADT was computed, the estimates were 44.3% (43.9% to 44.6%) and 25.5% (24.6% to 26.0%) for converted and control sites, respectively. In both cases, the difference in the percent reduction in the expected number of crashes during the years following the conversion was significantly larger at converted sites than at control sites, even though the number of crashes appears to decline over time at all sites. Results indicate that the reduction in the expected number of sites per mile has a steeper negative slope at converted than at control sites. Consistent with this, the forecasted reduction in the number of crashes per year and mile during the years after completion of the conversion at converted sites is more pronounced than at control sites. Seasonal effects on the number of crashes have been well-documented. In this dataset, we found that, as expected, the expected number of monthly crashes per mile tends to be higher during winter months than during the rest of the year. Perhaps more interestingly, we found that there is an interaction between the four to three-lane conversion and season; the reduction in the number of crashes appears to be more pronounced during months, when the weather is nice than during other times of the year, even though a reduction was estimated for the entire year. Thus, it appears that the four to three-lane conversion, while effective year-round, is particularly effective in reducing the expected number of crashes in nice weather.

Review of Alternative Distribution Methodologies for the Street Construction Fund of the Cities, December 26, 2003

Review of Alternative Distribution Methodologies for the Street Construction Fund of the Cities

Report of Recommendations to the Iowa Department of Human Services, Central Distribution Center, June 30, 2002

State Agency Audit Report

Report of Recommendations to the Iowa Department of Human Services - Central Distribution Center, June 30, 2003

State Agency Audit Report

Final Report, Secondary Road Fund Distribution Advisory Committee, 2002-2005

This report presents the findings and recommendations of the Secondary Road Fund Distribution Advisory Committee (SRFDAC) established by SF 2192 of the 2002 Iowa Acts.

Report of Recommendations to the Iowa Department of Human Services - Central Distribution Center, June 30, 2004

State Audit Reports - DHS Institutions

Report of Recommendations to the Iowa Department of Human Services - Central Distribution Center, June 30, 2005

Other Audit Reports - DHS Institutions

Audit report on the Iowa Department of Human Services – Central Distribution Center for the year ended June 30, 2006

Audit report on the Iowa Department of Human Services – Central Distribution Center for the year ended June 30, 2006

Report on the Iowa Department of Human Services – Central Distribution Center for the year ended June 30, 2007

Report on the Iowa Department of Human Services – Central Distribution Center for the year ended June 30, 2007

Final Report, Secondary Road Fund Distribution Advisory Committee, 2002-2005

This report presents the findings and recommendations of the Secondary Road Fund Distribution Advisory Committee (SRFDAC) established by SF 2192 of the 2002 Iowa Acts.

Report on the Iowa Department of Human Services – Central Distribution Center for the year ended June 30, 2008

Report on the Iowa Department of Human Services – Central Distribution Center for the year ended June 30, 2008

Notice on Civil Rights Obligations Applicable to the Distribution of Funds under the American Recovery and Reinvestment Act of 2009

Federal agencies will shortly begin distributing funding from the American Recovery and Reinvestment Act of 2009 (Recovery Act). They must do so in accordance with all nondiscrimination and equal opportunity statutes, regulations, and Executive Orders that apply to the distribution of funds under the Recovery Act. Agencies that grant funds also must ensure that their recipients and sub recipients comply with Title VI of the Civil Rights Act of 1964 (prohibiting race, color, and national origin discrimination including language access for limited English proficient persons), Section 504 of the rehabilitation Act of 1973 (prohibiting disability discrimination), Title IX of the Education Amendments of 1972 (prohibiting sex discrimination in education and training programs), the Age Discrimination Act of 1975 (prohibiting age discrimination in the provision of services), and a variety of program-specific statutes with nondiscrimination requirements.

Report on the Iowa Department of Human Services – Central Distribution Center for the year ended June 30, 2009

Report on the Iowa Department of Human Services – Central Distribution Center for the year ended June 30, 2009