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Home > Tools and Technology > Crash Reduction Factors

Toolbox of Countermeasures and Their Potential Effectiveness for Roadway Departure Crashes

Introduction

This issue brief documents estimates of the crash reduction that might be expected if a specific countermeasure or group of countermeasures is implemented with respect to roadway departure crashes and other non-intersection crashes. The crash reduction estimates are presented as Crash Reduction Factors (CRFs).

Traffic engineers and other transportation professionals can use the information contained in this issue brief when asking the following types of question: Which countermeasures might be considered along a particular section of a highway that is experiencing a high number of roadway departure crashes? What changes in the number of roadway departure crashes can be expected with the implementation of the various countermeasures?

When selecting countermeasures to reduce the number and/or severity of roadway departure crashes, the practitioner should first consider countermeasures designed to reduce the likelihood of vehicles leaving the roadway. Next, the practitioner should select strategies which minimize the likelihood of crashing into an object or overturning if the vehicle travels beyond the edge of the shoulder. Finally, the practitioner should consider countermeasures which reduce the severity of the crash such as improving the design and application of barrier and attenuation systems.

When selecting countermeasures to reduce the number and/or severity of crashes associated with hazardous roadside obstacles, the practitioner should refer to the AASHTO Roadside Design Guide which recommends these design options in order of preference:

1. Remove the obstacle;
2. Redesign the obstacle so it can be safely traversed;
3. Relocate the obstacle to a point where it is less likely to be struck;
4. Reduce impact severity by using an appropriate breakaway device;
5. Shield the obstacle with a longitudinal traffic barrier designed for redirection or use a crash cushion; and
6. Delineate the obstacle if the above alternatives are not appropriate.

Crash Reduction Factors

A CRF is the percentage crash reduction that might be expected after implementing a given countermeasure. In some cases, the CRF is negative, i.e. the implementation of a countermeasure is expected to lead to a percentage increase in crashes.

One CRF estimate is provided for each countermeasure. Where multiple CRF estimates were available from the literature, selection criteria were used to choose which CRFs to include in the issue brief:

• Firstly, CRFs from studies that took into account regression to the mean and changes in traffic volume were preferred over studies that did not.
• Secondly, CRFs from studies that provided aliitional information about the conditions under which the countermeasure was applied (e.g. road type, area type) were preferred over studies that did not.

Where these criteria could not be met, a CRF may still be provided. In these cases, it is recognized that the reliability of the estimate of the CRF is low, but the estimate is the best available at this time. The CRFs in this issue brief may be periodically updated as new information becomes available.

The Desktop Reference for Countermeasures lists all of the CRFs included in this issue brief, and alis many other CRFs available in the literature. A few CRFs found in the literature were not included in the Desktop Reference. These CRFs were considered to have too large a range or too large a standard error to be meaningful, or the original research did not provide sufficient detail for the CRF to be useful.

A CRF should be regarded as a generic estimate of the effectiveness of a countermeasure. The estimate is a useful guide, but it remains necessary to apply engineering judgment and to consider site-specific environmental, traffic volume, traffic mix, geometric, and operational conditions which will affect the safety impact of a countermeasure. The user must ensure that a countermeasure applies to the particular conditions being considered. The reader is also encouraged to obtain and review the original source documents for more detailed information, and to search databases such as the National Transportation Library (ntlsearch.bts.gov) for information that becomes available after the publication of this issue brief.

Presentation of the Crash Reduction Factors

In the Tables presented in this issue brief, the crash reduction estimates are provided in the following format:

CRF(standard error)^REF

The CRF is the value selected from the literature.

The standard error is given where available. The standard error is the standard deviation of the error in the estimate of the CRF. The true value of the CRF is unknown. The standard error provides a measure of the precision of the estimate of the true value of the CRF. A relatively small standard error indicates that a CRF is relatively precisely known. A relatively large standard error indicates that a CRF is not precisely known. The standard error may be used to estimate a confidence interval of the true value of the CRF. (An example of a confidence interval calculation is given below.)

The REF is the reference number for the source information.

As an example, the CRF for the countermeasure remove or relocate fixed objects outside of clear zone for all crashes is:

38(10)¹⁷

The following points should be noted:

• The CRF of 38 means that a 38% reduction in all crashes is expected after removing or relocating fixed objects outside of the clear zone.
• This CRF is bolded which means that a) a rigorous study methodology was used to estimate the CRF, and b) the standard error is relatively small. A CRF which is not bolded indicates that a less rigorous methodology (e.g. a simple before-after study) was used to estimate the CRF and/or the standard error is large compared with the CRF.
• The standard error for this CRF is 10. Using the standard error, it is possible to calculate the 95% confidence interval for the potential crash reduction that might be achieved by implementing the countermeasure. The 95% confidence interval is ±2 standard errors from the CRF. Therefore, the 95% confidence interval for removing or relocating fixed objects outside of the clear zone is between 18% and 58% (38 - 2×10 = 18%, and 38 + 2×10 = 58%).
• The reference number is 17 (Hovey and Chowdhury, as listed in the References at the end of this issue brief ).

Using the Tables

The CRFs for roadway departure crashes and other non-intersection crashes are presented in six tables which

summarize the available information. The Tables are:

• Table 1: Barrier Countermeasures
• Table 2: Bridge Countermeasures
• Table 3: Geometric Countermeasures
• Table 4: Median Countermeasures
• Table 5: Roadside Countermeasures
• Table 6: Signs/Markings/Operational Countermeasures

The following points should be noted:

• Where available, separate CRFs are provided for different crash severities. The crash severities are: all, fatal/injury, fatal, injury, or property damage only (PDO).
• Where available, road type information is provided.
• Where available, daily traffic volume (vehicles/day) is provided.
• Blank cells mean that no information is reported in the source document.
• For aliitional information, please visit the FHWA Office of Safety website (safety.fhwa.dot.gov).

Table 1: Barrier Countermeasures

Table 2: Bridge Countermeasures

Table 3: Geometric Countermeasures

Table 4: Median Countermeasures

Table 5: Roadside Countermeasures

Table 6: Sign/Marking/Operational Countermeasures

References

1. Agent, K. R., Stamatiadis, N., and Jones, S., “Development of Accident Reduction Factors.” KTC-96-13, Kentucky Transportation Cabinet, (1996)
2. Al-Masaeid, H. R. and Sinha, H., “An Analysis of Accident Reduction Potentials of Pavement Marking.” Journal of Transportation Engineering, ASCE, (1993) pp. 723-736.
3. Bahar, G., Mollett, C., Persaud, B., Lyon, C., Smiley, A., Smahel, T., and McGee, H., “NCHRP Report 518: Safety Evaluation of Permanent Raised Pavement Markers.”Washington, D.C., Transportation Research Board, National Research Council, (2004)
4. Bahar, G., Parkhill, M., Hauer, E., Council, F., Persaud, B., Zegeer, C., Elvik, R., Smiley, A., and Scott, B. “Prepare Parts I and II of a Highway Safety Manual: Knowledge Base for Part II”. Unpublished material from NCHRP Project 17-27. (May 2007)
5. Carrasco, O., McFadden, J., and Chandhok, P., “Evaluation of the Effectiveness of Shoulder Rumble Strips on Rural Multi-lane Divided Highways In Minnesota.” 83rd Transportation Research Board Annual Meeting, Washington D.C., (2004)
6. Chandler, B., “Eliminating Cross-Median Fatalities: Statewide Installation of Median Cable Barrier in Missouri.”TR News, No. 248, Washington, D.C., Transportation Research Board of the National Academies, (2007) pp. 29-31 .
7. Dudek, C. L., Richards, S. H., and Buffington, J. L., “Some Effects of Traffic Control on Four-Lane Divided Highways.”Transportation Research Record 1086, Washington, D.C., Transportation Research Board, National Research Council, (1986) pp. 20-30.
8. Elvik, R. and Vaa, T., “Handbook of Road Safety Measures.” Oxford, United Kingdom, Elsevier, (2004)
9. Gan, A., Shen, J., and Rodriguez, A., “Update of Florida Crash Reduction Factors and Countermeasures to Improve the Development of District Safety Improvement Projects.” Florida Department of Transportation, (2005)
10. Garder, P. and Davies, M., “Safety Effect of Continuous Shoulder Rumble Strips on Rural Interstates in Maine.” 2006 TRB 85th Annual Meeting: Compendium of Papers CD-ROM, Vol. TRB#06-2219, Washington, D.C., (2006)
11. Gates, T. J., Noyce, D. A., Talada, V., and Hill, L., “The Safety and Operational Effects of “Road Diet” Conversion in Minnesota.” 2007 TRB 86th Annual Meeting: Compendium of Papers CD-ROM, Vol. TRB#07-1918, Washington, D.C., (2007)
12. Griffin, L. I. and Reinhardt, R. N., “A Review of Two Innovative Pavement Patterns that Have Been Developed to Reduce Traffic Speeds and Crashes.” Washington, D.C., AAA Foundation for Traffic Safety, (1996)
13. Griffith, M. S., “Safety Evaluation of Rolled-In Continuous Shoulder Rumble Strips Installed on Freeways.”Transportation Research Record 1665, Washington, D.C., Transportation Research Board, National Research Council, (1999) pp. 28-33.
14. Gross, F. and Jovanis, P. P., “Estimation of Safety Effectiveness of Changes in Shoulder Width using Case-Control and Cohort Methods.” 2007 TRB 86th Annual Meeting: Compendium of Papers CD-ROM, Vol. TRB#07-2762, Washington, D.C., (2007)
15. Harwood, D. W., Council, F. M., Hauer, E., Hughes, W. E., and Vogt, A., “Prediction of the Expected Safety Performance of Rural Two-Lane Highways.” FHWA RD-99-207, McLean, Va., Federal Highway Administration, (2000)
16. Hirasawa, M., Takuda, T., Asano, M., and Saito, K., “Developing Optimal Centerline Rumble Strips and Evaluating Their Safety Benefits on National Highways in Hokkaido, Japan.” 2006 TRB 85th Annual Meeting: Compendium of Papers CD-ROM, Vol. TRB#06-1968, Washington, D.C. , (2006)
17. Hovey, P. W. and Chowdhury, M., “Development of Crash Reduction Factors.” 14801(0), Ohio Department of Transport, (2005)
18. Montella, A., “Safety Reviews of Existing Roads: a Quantitative Safety Assessment Methodology.” 2005 TRB 84th Annual Meeting: Compendium of Papers CD-ROM, Vol. TRB#05-1295, Washington, D.C., (2005)
19. Patel, R. B., Council, F. M., and Griffith, M. S., “Estimating the Safety Benefits of Shoulder Rumble Strips on Two Lane Rural Highways in Minnesota: An Empirical Bayes Observational Before-After Study.” 2007 TRB 86th Annual Meeting: Compendium of Papers CD-ROM, Vol. TRB#07-1924, Washington,
20. Persaud, B., Hauer, E., Retting, R. A., Vallurupalli, R., and Mucsi, K., “Crash Reductions Related to Traffic Signal Removal in Philadelphia.” Accident Analysis and Prevention, Vol. 29, No. 6, Oxford, N.Y., Pergamon Press, (1997) pp. 803-810.
21. Smith, E. B. and Ivan, J. N., “Evaluation of Safety Benefits and Potential Crash Migration Due to Shoulder Rumble Strip Installation on Freeways in Connecticut.” 2005 TRB 84th Annual Meeting: Compendium of Papers CD-ROM, Vol. TRB#05-1299, Washington, D.C., (2005)

    Updated August 2008

22. Harkey, D., Srinivasan, R., Baek, J., Council, F. M., Eccles, K., Lefler, N., Gross, F., Persaud, B., Lyon, C., Hauer, E., and Bonneson, J. A., “Crash Reduction Factors for Traffic Engineering and ITS Improvements,” NCHRP Report No. 617, (2008)
23. McMahon, P., Zegeer, C., Duncan, C., Knoblauch, R., Stewart, R., and Khattak, A., “An Analysis of Factors Contributing to ‘Walking Along Roadway’ Crashes: Research Study and Guidelines for Sidewalks and Walkways,” FHWA-RD-01-101, (March 2002)