Safety analysis will assist with making informed decisions on the type, deployment levels, and locations for safety countermeasures. This builds on the previous discussions on information sources to identify roadway departure issues. Regardless of implementation approach selected, some level of data analysis will be relevant. These analyses are most relevant for the identification and prioritization of locations with safety issues and selection of appropriate countermeasures for the spot location approach, for the countermeasure-based systematic approach, the safety analysis discussed in this section would occur only after the selection of proven low cost countermeasures. In the crash type-based systematic approach, analysis will focus only on those roadway departure crash types identified as pertinent in the local jurisdiction.
Crash data analysis is used to determine the extent of the roadway departure safety issue, the priority for the application of scarce resources, and selection of appropriate countermeasures. The two main quantitative analysis methods for roadway departure crashes are crash frequency and crash rates.
Crash frequency is defined as the number of crashes occurring within a determined study area. A local practitioner can determine crash counts using information compiled from the State crash database or law enforcement crash reports. This will allow the practitioner to:
- Summarize the crashes by attributes such as type, severity and location;
- Spatially display the sites on a map using push pins or a GIS software package;
- Provide a report sorted by location and crash type to identify problem locations;
- Determine predominant roadway departure crash types and associated roadway physical characteristics; and
- Determine appropriate countermeasures.
Once this information is collected and displayed, the practitioner can complete a methodical analysis by county or route, and also a cluster analysis to determine those roadway locations that have experienced a high or moderate level of crashes.
Crash rates can be an effective tool to measure the relative safety at a particular location. The calculation of crash frequency (crashes per year) divided by vehicle exposure (traffic volumes, or roadway length) results in a crash rate. Crash rate analysis can be a useful tool to determine how a specific roadway or segment compares to an average roadway on the network. A count of the number of crashes is often inadequate when comparing multiple roadways of varying lengths and/or traffic volume. Crash rate is often used to prioritize locations for safety improvements when working with limited budgets and trying to achieve the greatest safety benefits with limited resources.
For example, it is possible that two roadways in a jurisdiction (Route A and Route B) each have the same number of crashes. However, Route A could have more than double the number of vehicles on a typical day than does Route B. To effectively compare the relative safety of the two locations, the practitioner must factor in the level of exposure on each route. Exposure is often represented by number of vehicles using the route or by the length or roadway.
One limitation of crash rates for low volume roads is the sensitivity of the formula to low traffic volume. The crash rate calculation is not as beneficial at low volumes as it is with higher volume roads, as small changes in the number of vehicles results in a disproportionate change in the crash rate for the segments that in reality operate similarly.
Where traffic volume data is unavailable, other information can be used to provide exposure information. One often-used factor is the length of the roadway segment on each route studied. Comparing the number of roadway departure crashes per mile can help an agency identify potential opportunities to improve safety.
Appendix C includes formulas for calculating crash rates on roadway segments and examples of crash rate calculations by vehicle miles traveled and by roadway mileage.
Qualitative analysis considers the physical characteristics of the identified sites. This can take the form of examination of maps and photographs or field assessment.
As discussed in Section 2, field assessments can be an informal review of the safety of the roadway, or be more formalized in a Road Safety Audit. The RSA will qualitatively estimate and report on existing and potential roadway departure safety issues and identify opportunities for improvements. In the case of the countermeasure-based systematic approach, the field assessment can be used to validate the countermeasures selected.
During the field assessment, it is important to determine if identified locations comply with the minimum standards for signs and pavement markings included in the Manual on Uniform Traffic Control Devices (MUTCD). The MUTCD provides the minimum standard requirements for traffic control devices on all public streets, highways, bikeways, and private roads open to public travel.8 Complying with these requirements can assist in improving safety on the transportation system. Local practitioners should also contact the State DOT for the State's minimum requirements, as often they are more stringent than the requirements in the MUTCD.
If the traffic control devices on the segment are not in compliance with MUTCD or the State minimum requirements, appropriate devices should be installed. Non-compliance is an important consideration that can affect road safety and might have liability implications for a jurisdiction.
Figure 2 shows some of the most common traffic control devices related to roadway departure treatments.
Figure 2. MUTCD Traffic Control Devices
- Speed Limit. Speeding is often a contributing factor in roadway departure crashes. Posting appropriate speed limit signs along a roadway provides motorists with guidance for safe driving speeds.
- Reduced Speed Ahead. When the speed limit is reduced (for example, when entering the city limits of a city, town or village), the Reduced Speed Ahead sign can provide motorists warning of that change.
- Curve Warning Signs
- Curve/Turn and Reverse Curve/Turn. These horizontal alignment signs are the basic treatment for a horizontal alignment change. Care should be taken to choose the most appropriate sign and to place it at the optimum location for approaching motorists.
- Advisory Speed Plaque. The advisory speed plaque provides drivers with additional information regarding the relative sharpness of an approaching curve or turn.
- Chevron Alignment Signs. These signs are placed within the curve or turn to provide delineation and guidance to motorists as they drive through the curve. Chevron signs are particularly helpful with combination horizontal/vertical curves, as the vertical curvature can hide the horizontal alignment change from view of motorists. The delineation provided by chevrons can help guide drivers through this complex alignment.
- One-direction Large Arrow. The large arrow sign is used to supplement curve or turn signs to provide motorists additional warning within the curve or turn.
- Object Markers (Type 1, 2, 3). Object markers are used to warn drivers of obstructions within the roadway or adjacent to the roadway (e.g., culvert, other roadside obstacle).
No Passing Zones
- Pavement Marking. No Passing Zones are marked at locations where drivers do not have sufficient sight distance to safely overtake a slower moving vehicle. The solid yellow line on the motorist's side of the center line is an indication that the motorist should wait for a safe passing opportunity.
- No Passing Zone Pennant Sign. The No Passing Zone pennant sign supplements the pavement marking to further emphasize the sight distance restrictions.
8 The MUTCD can be found at http://mutcd.fhwa.dot.gov. Local practitioners should be cognizant that many States adopt standards that are more stringent than the MUTCD. Where these exist, the local practitioner is required to comply with those State standards. This information can be obtained from the State Department of Transportation or their LTAP Center.