Guest post by Jonah Finkelstein, EIT Spack Consulting
Driver and pedestrian safety is one of the most important considerations during planning and evaluation of a roadway network. A crash analysis is an integral part of a safety analysis and are often completed at intersections and corridors. The crash analysis can help assess concerns and help determine potential improvement options. Before we can jump into the steps needed to complete intersection and corridor crash analyses, let’s review the tools and other important information associated with this type of crash analysis. These definitions are based on the process in Minnesota, but other states should have similar tools and information for your use.
- MnCMAT. Minnesota’s Crash Mapping Analysis Tool. This tool for Minnesota that compiles and posts crash information, as reported to the Department of Public Safety. A username and password is required (Additional information, user manual, Webinar training, and access can be requested through the following link – http://www.dot.state.mn.us/stateaid/crashmapping.html).
- Intersection Green Sheets. Short-hand reference to the Minnesota Department of Transportation’s statewide average crashes on various types of intersections. These averages are used as benchmark for comparison as well as to determine critical crash rates.
- Section Green Sheets. Short-hand reference to the Minnesota Department of Transportation’s statewide average crashes on various types of sections or corridors. These averages are used as benchmark for comparison as well as to determine critical crash rates. These rates are presented with all crashes included intersections and with only those crashes not related to intersections.
- Intersection Crash. Although seemingly simple, the definition of an intersection crash is a little more complicated than expected. Any crash within the boundaries of an intersection itself is always considered an intersection crash. However, crashes that occur 100-, 200-, or more feet away from the intersection may or may not be related to the intersection.
Typically, we will start with a 250-foot radius around the center of the intersection and then do a quick review to determine if all crashes are related to the intersection. For example, a rear end crash that occurs 150 feet from an intersection is most likely related. A run-off the road heading away from the intersection is not.
- Crash Severity. The seriousness of a crash and its impact on the drivers and occupants. Typically, crashes are sub-divided into five categories:
- Fatal Crash – Crash Type K, deaths that occur as a result of the crash.
- Incapacitating Injury – Crash Type A, injuries serious enough to prevent normal activity for at least one day, such as massive blood loss, broken bones, etc.
- Non-incapacitating Injury – Crash Type B, injuries that are evident at the scene, but not serious enough to prevent normal activity, such as cuts, bruises, limping, etc.
- Possible Injury – Crash Type C, non-visible injuries but there are complaints of pain or momentary unconsciousness, such as headaches, etc.
- Property Damage – Crash Type PD or PDO, no injuries as a result of the crash.
- Crash Density. The number of crashes at an intersection or on a corridor divided by the number of years over which those crashes occurred.
- Roadway Characteristics for Intersection
- Entering volume – total entering volume of all approaches
- TCD – Traffic Control Device
- GEN – Environment, Urban, suburban, City Bypass, or Rural
- Maximum SL – posted Speed Limit
- Volume on Highest Leg – Approach with highest ADT
- Roadway Characteristics for Corridor
- Length – Length of corridor in miles
- Volume (ADT) – Corridor ADT
- ENV – Rural, Urban, Suburban
- DSG – Design, Freeway, expressway, conventional (other)
- LNS – Number of lanes
- MED – Median Type
- Observed Crash Rate. The measured number of crashes controlling for exposure (traffic volume) to allow an ‘apples-to-apples’ comparison between intersections or between corridors. Typically presented in a crash number per one million entering vehicles (intersections) or one million vehicle miles traveled (VMT, for corridors).
- Critical Crash Rate. Provides a statistical threshold for screening site and is calculated by weighting the average crash rate for a similar intersection or segment by the existing volume. This is a reliable way to determine if the number of crashes are above the statistical range of crashes that could occur and indicative of a potential safety concern or issue.
- Critical Index. The ratio of the observed crash rate to the critical crash rate. A critical index exceeding 1.00 indicates a potential safety concern. A critical index of 1.00 or less indicates performance within expectations without deviation from statewide trends.
- The fatal and incapacitating injury crashes (Type A) combined. When calculated into an FAR Crash Rate, defined as the number of crashes per 100 million entering vehicles (intersections) or 100 million vehicle miles traveled (VMT, for corridors).
A final point to keep in mind, please use the term ‘crash’ instead of ‘accident’. An accident implies that it could not be avoid or happened by chance. A crash simply implies a collision of some type. Since we don’t want to play the blame game, referring to the impact or crash only is our preferred language.
