Chapter 7 – Added reference to Pedestrian. Streetscape Guide and Public Rights-of-Way. Accessibility Guidelines manual. Chapter 9 – Added. Manual for Streets (MfS – Department for Transport, Communities & Local Government and Welsh Assembly Government, March 2007) has captured the output from this research to examine: a) the relationship between SSD, street widths and vehicles speeds, and b) more appropriate levels of visibility.
Intersection Safety: A Manual for Local Rural Road Owners3. Safety AnalysisConducting safety analyses will assist the practitioner in identifying intersections with safety issues and selecting countermeasures to improve them. The types of analysis can be qualitative or quantitative. This section outlines steps to identifying intersections with safety issues and making data-supported decisions as to the type, deployment levels, and locations of countermeasures.
These steps build on the previous discussion of overall safety implementation approaches and sources of information for identifying safety problems. Additional information on analysis procedures and data can be reviewed in 'Road Safety Information Analysis: A Manual for Local Rural Road Owners.' 3.1.Crash FrequencyCrash frequency represents the number of crashes that have occurred at a particular intersection over a period of time. It can be determined from the State or local crash database (or law enforcement crash reports).This allows the practitioner to:. Summarize the crashes by type and location;. Spatially display crash locations on a map using push pins or a computer software package; and. Provide a report identifying intersections with a history of crashes.Once this information is collected and displayed, the local practitioner can compare intersections using cluster analysis to determine crash experience by frequency levels.
3.2.Crash RateCrash frequency alone is often inadequate when comparing multiple intersections or prioritizing locations for improvement. Crash rates can be an effective tool to measure the relative safety at a particular intersection. The ratio of crash frequency (crashes per year) to vehicle exposure (number of vehicles entering the intersection) results in a crash rate. Crash rate analysis can be a useful tool to determine how a specific intersection compares to the average intersection on the roadway network.For example, it is possible that two intersections in a jurisdiction (Intersection A and Intersection B) each have a similar number of crashes. However, Intersection A may have many more vehicles entering the intersection on a typical day than Intersection B, as shown in Table 2. In order to effectively compare the safety of the two locations, the practitioner must factor in the level of exposure to crashes for each intersection.
Exposure data here is represented by the number of vehicles entering the intersection. Population and number of licensed drivers within a jurisdiction are other types of exposure data that can be used depending on the circumstances and availability.Crash rate is often used to prioritize locations for safety improvements when working with limited budgets to achieve the greatest safety benefits with available resources.Crash rates can be calculated using the following widely accepted equation.
This equation can be used for any crash type or severity. The intersection crash rate based on vehicles entering the intersection is calculated as:Where:R = Crash rate for the intersection expressed as crashes per million entering vehicles (MEV)C = Total number of intersection-related crashes in the study periodN = Number of years of dataV = Traffic volumes entering the intersection dailyThis equation relies on traffic volume information. Actual and estimated traffic volumes are often compiled and kept by State highway agencies, local governments and property developers.In the following example shown in Table 2, two intersections have approximately the same number of crashes but different entering traffic volumes. By factoring in traffic volume (exposure), the calculation indicates that Intersection B may be a more promising roadway for safety treatments due to its higher intersection crash rate (measured in number of crashes for every 1 million entering vehicles).