Paved shoulders

Paved shoulders are treatments that promote safe recovery for vehicles that have left the road. In Canada in 2011, 19.1% of all crash victims that were killed and 10.8% of those that were injured suffered their injuries in single vehicle occurrences where vehicles ran off the left- or right-shoulder of the roadway (Transport Canada, 2013). The probability of a safe recovery for a vehicle that has left its lane and entered the shoulder area is increased if the vehicle has a smooth area in which to initiate a recovery and if the recovery. A pavement irregularity that causes the driver to either fail to re-enter the lane or to enter it at such an angle that the vehicle crosses into the opposing lane, would impede a safe re-entry.

A study that used generalized linear models to investigate the relationship between crash reduction and the implementation of paved shoulders was carried out in Iowa. Ten years after implementation, the results of the study demonstrated that total crashes decreased by 15.9% on road sections with paved shoulders. Sites with paved shoulders also had 13.5% fewer run off road crashes than control sites. Single vehicle run off road crashes were 16.4% lower on road sections with paved shoulders than at sites that had not been treated (Hallmark et al, 2010).

In 2006, the Alabama Department of Transportation began installing 2-ft of paved shoulders on both sides of the highways during resurfacing and reconstruction projects. A benefit-cost analysis was carried out on the installations that occurred in the two years following the introduction of this countermeasure. Road sections that had the highest crash and severity rates were identified as high priority treatment segments that would benefit significantly from reduced collisions, if this treatment were implemented. The resulting benefit-cost ratios for the high priority segments ranged from 0.08:1 to 2.20:1 for the segments identified based on crash rate and 1.22:1 to 8.18:1 for those segments identified based on severity (Ayala, 2008).

Zegeer et al., (1988) conducted a study to determine the effect of lane widening, shoulder widening, and shoulder surfacing on traffic collision frequency. Detailed traffic, collision, roadway, and roadside data were collected on 4,951 miles of two-lane roadway in seven states. A collision prediction model and statistical testing were used to determine the expected crash reductions related to the geometric improvements. Paved shoulders widths between zero and 12 ft. that were widened by 2 ft., 4 ft., and 6 ft., respectively, were expected to experience related collision reductions of 16%, 29% and 40%, respectively.

An evaluation of the safety effectiveness of composite shoulders, wide unpaved shoulders, and wide paved shoulders in Kansas revealed that upgrading narrow unpaved shoulders to composite shoulders, consisting of three feet of pavement with the remainder aggregate or turf, could reduce shoulder related crashes by up to 61% and fatal and injury crashes by 31%. The authors of the report (Zeng et al., 2013) also found that wide paved shoulders provided more safety benefits than composite shoulders.

Stamatiadis et al., (2009) conducted research using crash data from California, Kentucky, and Minnesota to quantify the safety and operational impacts of design element (lane-width, shoulder-width, and median type and width) trade-offs and their associated risks. The authors estimated that widening paved shoulders from 3 to 8 ft. could reduce total crashes by 29% and single vehicle occurrences by 23%. However, the authors also found that while wider paved shoulders may contribute to improved safety, they may also contribute to increased vehicle operating speeds and increased crash severity.

Scope of the Problem

• Transport Canada (2013). National Collision Data Base. Retrieved from http://wwwapps2.tc.gc.ca/Saf-Sec-Sur/7/NCDB-BNDC/p.aspx?l=en

Evidence

• Hallmark, S.H., McDonald, T.J., Tian, Y., and Andersen, D.J. (2010). Safety Benefits of Paved Shoulders. (Report No. 0-5-239). Iowa Department of Transportation, Ames, IA. Retrieved from http://www.intrans.iastate.edu/reports/updated_paved_shoulders_w_cvr1.pdf
• Ayala, R. (2008). A Benefit/Cost Analysis of Paved Shoulder Installation on High Priority Road Segments on Two-Lane Rural Highways in Alabama. Auburn University. Retrieved from http://etd.auburn.edu/etd/handle/10415/1158
• Zegeer, C.V., Reinfurt, D.W., Hummer, J., Herf, L., and Hunter, W. (1988). Safety Effects of Cross-section Design for Two-Lane Roads. Transportation Research Record, Issue No. 1195, p. 20-32. Washington, D.C. Retrieved from http://trid.trb.org/view.aspx?id=302144
• Zeng, H., Schrock, S.D., Mulinazzi, T.E. (2013). Evaluation of Safety Effectiveness of Composite Shoulders, Wide Unpaved Shoulders, and Wide Paved Shoulders in Kansas. (Report No. K-TRAN: KU-11-1). Kansas Department of Transportation, Topeka, KS. Retrieved from http://www.kutri.ku.edu/research/k-trans/2011_SteveSchrock_Evaluation_Incremental_Paved_Shoulder_Width_Safety/KU-11-1_Final.pdf
• Stamatiadis, N., Pigman, J., Sacksteder, J, Ruff, W., and Lord, D. (2009). Impact of Shoulder Width and Median Width on Safety. NCHRP Report 633, Transportation Research Board, Washington D.C. Retrieved from http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_rpt_633.pdf