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                        Roadway Departure Safety: A Manual for Local Rural Road Owners


            
            In This Section
        Local and Rural Road Safety Program
                Training, Tools, Guidance and Countermeasures for Locals Practitioners
                Crash Facts
                
                Appendix C: Crash Rate Calculations
The crash rate for roadway departure crashes on a roadway is calculated as:

"Formula: R equals C x 100,000,000 divided by V x 365 x N x L"

The variables in this equation are:

R = Roadway Departure crash rate for the road segment expressed as crashes per 100 million vehicle-miles of travel,
	C = Total number of roadway departure crashes in the study period
	V = Traffic volumes using Average Annual Daily Traffic (AADT) volumes
	N = Number of years of data
	L = Length of the roadway segment in milesThis equation relies on having traffic volume information To determine how to obtain actual and estimated traffic volumes for a particular roadway, a local agency can contact its State highway agency, LTAP representative, or other state agencies.

**Example 1. Crash Rate by Vehicle Miles Traveled**

In this example, two roadways have the same number of crashes but different traffic volumes. By factoring in exposure, the calculation indicates that Route B may be more susceptible to future crashes. However, before any decision is made, other factors such as roadway geometrics, cross section, and other potential differentiating factors should be considered. There could be other issues not related to traffic volume that affect crash rates.



Table 3. Example of Roadway Departure Crash Rate Calculation by Vehicle Miles Traveled
	Roadway
			RD Crashes (C)
			Traffic Volume (V)
			Years of Data (N)
			Length of segment (L)
			Crash Rate (R)Route A
			15
			4,000
			5
			12 miles
			17.1Route B
			15
			2,500
			5
			12 miles
			27.4Route A has experienced 17.1 crashes per 100 million vehicle-miles traveled on that roadway. Route B has experienced 27.4 crashes per 100 million vehicle-miles traveled. This data can be used to compare the two roadways. In this case, even though both routes had the same number of crashes, Route B is more susceptible to crashes based on the level of exposure. The practitioner could consider Route B a more promising candidate for a safety treatment than Route A due to its higher crash rate.

**Example 2. Crash Rate by Route Length**

In this example, two roadways have the same number of crashes but different roadway lengths. Traffic volume data is not available.

A "crashes per mile" rate for road segments is calculated as:



Where:

**R** = Crashes per mile for the road segment expressed as crashes per each 1 mile of roadway per year.
	**C** = Total number of crashes in the study period.
	**N** = Number of years of data.
	**L** = Length of the roadway segment in miles.

Table 4. Example of Roadway Departure Crash Rate Calculation by Route Length
	Roadway
			RD Crashes (C)
			Years of Data (N)
			Length of Segment (L)
			Crashes per Mile (R)Route A
			12
			5
			17 miles
			0.14Route B
			12
			5
			26 miles
			0.09In this example, Route A has experienced 0.14 crashes per roadway mile. Route B has experienced 0.09 crashes per mile of roadway. In this case, even though both routes have the same number of crashes, Route A may be more susceptible to future crashes. Therefore Route A may be a more promising candidate for safety treatments.
      
  Book traversal links for Appendix C: Crash Rate Calculations
      ‹ Appendix B: 2009 MUTCD Standards for Traffic Control Devices
                    Up
                
                Book:Roadway Departure Safety: A Manual for Local Rural Road Owners
              1. Introduction and Purpose
                2. Identifying Safety Issues
                3. Safety Analysis
                4. Countermeasures
                5. Evaluation
                6. Case Studies
                7. Summary
                Appendix A: Additional Resources
                Appendix B: 2009 MUTCD Standards for Traffic Control Devices
                Appendix C: Crash Rate Calculations U.S. DEPARTMENT OF TRANSPORTATION

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