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7. Appendices

Road Diet Informational Guide

Appendix A – Road Diet Safety Assessment Studies

The following table provides an overview of recent Road Diet safety analyses, including the number of treatment sites, traffic volume, and key safety results. Following that are synopses for each reference.

ReferenceTreatment SitesADTKey Safety Results
FHWA, 201045 sites in California, Iowa, and Washington3,718 to 26,376Iowa data:  47% reduction  in total crashes
California and Washington data: 19% reduction in total crashes
Combined data: 29% reduction in total crashes
Noyce et al., 20067 treatment sites throughout Minnesota8,900 to 17,400Traditional before-after approach: 42- 43% reduction in crashes.
Yoked/group comparison analysis: 37% reduction in total crashes and 47% reduction in crash rates.
EB approach: 44% reduction  in total crashes.
Pawlovich et al., 200615 treatment sites throughout Iowa4,766 to 13,69525.2% reduction in crash frequency per mile; 18.8% reduction in crash rate.
Li and Carriquiry, 200515 treatment sites throughout Iowa3,007 to 15,33329% reduction  in the frequency of crashes per mile; 18% reduction in the crash rate.
Huang et al., 200312 treatment sites in California and Washington10,179 to 16,0706% reduction  in total crashes relative to control; no reduction in crash rate.
Lyles et al., 201224 treatment sites throughout Michigan3,510 to 17,0209% reduction  in total crashes (non- significant).
Stout, 2005; Stout et al., 2005; Stout (year unknown)11 to 15 treatment sites in various Iowa cities2,000 to 17,40021 to 38 percent reduction in total crashes; similar reduction in crash rates.
Clark, 2001One treatment site in Athens-Clarke County, GA18,000 to 20,00052.9% reduction in total crashes; 51.1% reduction in crash rate (first 6 months).
City of Orlando, 2002One treatment site in Orlando, FL18,000 to 20,00034% reduction  in crash rate; 68% reduction in injury rate (first 4 months).
Preston, 1999MinnesotaNot Provided27% lower crash rate on three-lane roads than on four-lane undivided roadways (cross-sectional comparison – not a before-after study)

The table below provides additional details for these Road Diet safety assessments.

ReferenceFHWA. 2010. Evaluation of Lane Reduction "Road Diet" Measures on Crashes. FHWA Report No. FHWA- HRT-10-053.
Location45 treatment sites in California, Iowa, and Washington
ADT3,718 – 26,376
Safety Analysis MethodThe empirical Bayes (EB) methodology was used to estimate the change in total crashes.
Reported Safety EffectsThe EB evaluation of total crash frequency indicated a statistically significant effect of the Road Diet treatment in both data sets and when the results are combined. The Iowa data indicate a 47% reduction  in total crashes while the California and Washington data indicate a 19% decrease. Combining both data sets results in a 29% reduction  in total crashes.

This is arguably the strongest crash-based evaluations of Road Diet implementation.

Two likely reasons the results differ from the original Iowa results (below) is that the re-analysis involved a much larger reference group than was used in the original study, and the re-analysis provided  more weight to longer sites (while the original study weighted all treatment sites equally regardless of length).

Differences between the IA sites and those in CA/WA may be a function of traffic volumes and characteristics of the urban environments where the Road Diets were implemented. AADT for the IA sites ranged from 3,718 to 13,908 and were predominately on U.S. or State routes passing through  small towns; AADT for the sites in CA and WA ranged from 6,194 to 26,376 and were predominately on corridors in suburban environments that surrounded larger cities.

Sites with lower crash reduction  factors (CRFs) generally had higher traffic volumes, suggesting the possibility of diminishing safety benefits as traffic volumes increase.

The authors recommended that the choice of which CRF to use should be based on characteristics of the site being considered. If the proposed treatment site is more like the small-town Iowa sites, then the 47% reduction found in IA should be used. If the treatment site is part of a corridor in a suburban area of a larger city, then the 19% reduction should be used. If the proposed site matches neither of these site types, then the combined 29% reduction  is most appropriate.


ReferenceNoyce, D.A.; Talada, V.; and Gates T.J. 2006. Safety and Operational Characteristics of Two-Way Left-Turn Lanes. Minnesota DOT Report No. MN/RC 2006-25.
Location7 treatment sites throughout Minnesota
ADT8,900 – 17,400
Safety Analysis MethodCrash data were first analyzed using traditional approaches involving  a comparison of the before and after crashes. Crash data were also analyzed by yoked/group comparison analysis and the empirical  Bayes (EB) approach.
Reported Safety Effects

The traditional before-and-after approach estimated a reduction  in total crashes between 42 and 43%.

A yoked/group comparison analysis found  a 37% reduction  in total crashes and a 46% reduction in PDO crashes (both statistically significant). The reductions in crash rates (per vehicle mile traveled) were 47% for total crashes and 45% for PDO crashes (both statistically significant).

The empirical Bayes (EB) approach estimated a 44% reduction  in total crashes.

CommentsThis is one of the stronger crash-based evaluations of Road Diet implementation, although the number of treatment sites (7) is small.  One limitation of the authors' use of the empirical Bayes (EB) approach involves the relatively small group of reference sites (17).  By comparison, the EB analysis by FHWA (2010) summarized 296 reference sites.


ReferencePawlovich, M.D.; Li, W.; Carriquiry, A.; and Welch, T.M. 2006. Iowa's Experience with "Road Diet" Measures: Impacts on Crash Frequencies and Crash Rates Assessed Following a Bayesian Approach.  TR Record Issue Number 1953
Location15 treatment sites throughout Iowa
ADT4,766 to 13,695
Safety Analysis MethodA before-and-after study implemented from a Bayesian perspective to assess crash history effects. The study used both monthly  crash data and estimated volumes over 23 years (1982 to 2004). Crash data were analyzed at each site before and after the conversions were completed.
Reported Safety EffectsResults indicate a 25.2% (23.2% to 27.8%) reduction in crash frequency per mile and an 18.8% (17.9% to 20.0%) reduction in crash rate. The values in parentheses represent the 95% confidence interval.

This is a relatively  strong crash-based evaluation of Road Diet implementation.  The methodology is a refinement from the 2005 study by Li and Carriquiry.

Unlike the use of linear regression models to estimate expected crash frequencies, this study allowed for different slopes during the "before" and the "after" periods by including a change-point in the model and for the interaction of treatment and slope. As a result, the model allows for a slight increase in crash frequency during the months immediately preceding and following the conversion.

The number of comparison sites (15) is much smaller than the number  of reference sites (296) used in the EB
analysis performed by FHWA (2010).


ReferenceLi, W. and Carriquiry, A. 2005. The Effect of Four-Lane to Three-Lane Conversion on the Number of Crashes and Crash Rates in Iowa Roads. Department of Statistics, Iowa State University.
Location15 treatment sites throughout Iowa
ADT3,007 – 15,333
Safety Analysis MethodThe authors assessed the effectiveness of the four to three lane conversion by comparing the average expected annual crash frequency  per mile during  years preceding and following the conversion at the site level and also as an average over all sites in each of the two groups (Road Diets and comparison sites).
Reported Safety Effects

In general, with elapsed time, the expected number of crashes per mile at each site in the treatment group continues to decrease faster than the number at the corresponding paired site in the control group.

For all treatment sites combined, the frequency of crashes per mile decreased an estimated 34.8%, from 23 pre-treatment to 15 post-treatment, whereas the crash frequency per mile for control sites decreased 6.2%, from 16 pre to 15 post. This would  suggest an estimated 29% net reduction  in the frequency of crashes per mile associated with the Road Diet treatments.

For all treatment sites combined, the annual crash rate per 100MVMT decreased an estimated 43.9%, from 792 pre-treatment to 442 post, whereas the crash rate for control sites decreased 25.5%, from 652 pre to 486 post. This would  suggest an estimated 18% net reduction in the crash rate per 100MVMT associated with the Road Diet treatments.

CommentsWhile the results suggest that traffic safety is significantly improved by converting four lane roads to three lanes, there was significant variability in crash numbers across sites. It is not clear how much of an impact the wide range in ADT (3,007 – 15,333) had on the overall safety analysis. The suitability of the control sites may be questionable given markedly lower crash frequencies and crash rates at the control sites compared with the treatment sites, pre-intervention.


ReferenceHuang, H.; Stewart, J. R.; Zegeer, C.; and Tan Esse; C. 2003. How Much Do You Lose When Your Road Goes on a Diet? Submitted to the 2nd Urban Street Symposium.
Location12 treatment sites in California and Washington
ADT10,179 to 16,070 pre-conversion
Safety Analysis MethodThe authors conducted before-and-after analysis using a yoked comparison study of the Road Diet and comparison sites. Further analysis used a negative binomial model controlling for possible changes in ADT, study period, and other factors.
Reported Safety EffectsAfter accounting for trends at comparison sites, the number of crashes at Road Diet sites in the after period declined by about 6%. Crash rates, however, did not change significantly from the "before" period to the "after" period.
CommentsAlthough the authors identified 30 Road Diets and 50 comparison sites in 8 cities, it is unclear why only 12 treatment sites and 25 comparison sites were included in this paper. ADTs were not available for some treatment and comparison sites, and some of the ADTs were of "questionable accuracy." The selection of comparison sites is a key function of the yoked comparison study design, and little information is provided regarding the criteria used to select comparison sites.


ReferenceLyles, R.; Siddiqui, M.A.; Taylor, W.; Malik, B.; Siviy, G.; and Haan, T. 2012. Safety and Operational Analysis of four- lane to three-lane Conversions (Road Diets) in Michigan. Michigan DOT Report Number RC-1555
Location24 treatment sites throughout Michigan
ADT3,510 – 17,020
Safety Analysis MethodSimple before-and-after crash analysis adjusted for trends of an untreated comparison group.
Reported Safety EffectsAverage CMFs, adjusted for citywide trends, were calculated across all 24 sites. The result was that the overall naïve (unadjusted) CMF was estimated as 0.63, and 0.91 after adjustment. While the best estimate of a usable CMF is 0.91, this is not statistically different from 1.0 and is an average across all sites. Perhaps more importantly, there is a great deal of variation from site to site.
CommentsThe analysis was limited by the fact that good/acceptable comparison sites could be identified for only a few of the 24 sites. The authors caution that Road Diets should not be "oversold" with respect to expected benefits, especially safety benefits. Actual benefits of a Road Diet can vary significantly by site.



Stout, T.B. 2005. Before and After Study of Some Impacts of Four-lane to Three-lane Roadway Conversions. Unpublished paper: Iowa State University.
Stout, T.B; Pawlovich, M.; Souleyrette, R.R.; and Carriquiry, A.  2005. Safety Impacts of "Road Diets" in Iowa. Unpublished paper: Iowa State University.
Stout, T.B. Year unknown. Matched Pair Safety Analysis of Four-Lane to Three-Lane Roadway Conversions In Iowa. Unpublished paper: Iowa State University.

LocationVarious Iowa cities
ADT2,000 – 17,400
Safety Analysis MethodBefore-and-after study using yoked comparison pairs and a comparison to the cities in which the sites were located.
Reported Safety EffectsThe three sets of analyses examined before-and-after changes at largely the same group of converted sites, with some additional locations added with the passage of time. The studies reported reductions in crash frequency that ranged from 21 to 38 percent. The studies reported somewhat similar reductions in crash rates, as well as reductions in the numbers of crashes related to left turns and stopped traffic.

The studies reported a greater difference in crash reduction  between  the study segments and the yoked segments than was found between the study segments and the citywide data, which the author(s) attributed to greater variation in the changes in crashes in the yoked segments. The implied degree of effectiveness for the yoked comparison was larger than for the citywide comparisons, and according to the author, might be an artifact of the selection of the yoked segments.

The methodology did not account for possible regression-to-mean effects, and no tests of statistical significance were provided.


ReferenceClark, D.E. 2001. Road Diets: Athens-Clarke County's Experience in Converting Four-lane Roadways into Three-lane Roadways. Washington DC. Proceedings of the Institute of Transportation Engineers Annual Meeting.
LocationOne treatment site in Athens-Clarke County, GA
ADT18 – 20K
Safety Analysis MethodSimple before-and-after
Reported Safety EffectsDuring the first 6 months after the change in lane configuration there were 40 reported crashes along the treated corridor compared with 85 crashes during the same 6 month  period for the previous year. That corresponds to a 52.9% reduction. Crashes per million vehicles declined 51.1%, from 19.74 to 9.65.
CommentsThe results of this study support other studies that show safety benefits associated with Road Diet implementation, but the relatively short post-intervention period and the lack of robust safety analysis methodology limit the utility of these findings.


ReferenceCity of Orlando. 2002. Edgewater Drive Before & After Re-Striping Results. City of Orlando - Transportation Planning Bureau.
LocationOne treatment site in Orlando, FL
ADT18 – 20K
Safety Analysis MethodSimple before-and-after
Reported Safety EffectsDuring the first 4 months after the change in lane configuration the annualized crash rate per MVM declined 34%, from 12.6 (for 3 years preceding implementation) to 8.4.  The injury rate per MVM declined 68%, from 3.6 to 1.2 (for the same time periods).
CommentsThe results of this study support other studies that show safety benefits associated with Road Diet implementation, but the relatively short post-intervention period and the lack of robust safety analysis methodology limit the utility of these findings.


ReferencePreston, H.  1999. Access Management – A Synthesis of Research. Report MN/RC – REV 1999-21. Minnesota Department of Transportation.
Safety Analysis MethodThis was not a before-and-after study.  The author presents a simple cross-sectional comparison using 1991- 1993 statewide crash data.
Reported Safety EffectsThe crash rate per Million VMT for urban four-lane undivided roads was 6.75 versus a crash rate of 4.96 for three-lane roads.  This comparison suggests that three-lane roads have a crash rate that is 27% lower than the rate for four-lane undivided roadways.
CommentsThe number of miles of three-lane roads was small – 14 miles, versus 299 miles of four-lane undivided roads. The simple cross-sectional comparison does not take into account many confounding factors such as speed limits, pedestrian activity, land use, intersection spacing, driveway access, etc.

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