When a location has been determined to be a high crash location, the next step in the safety management process is to diagnose the cause(s). This involves developing an understanding of crash patterns and the physical characteristics and road user behaviours which contribute to these crashes. This diagnosis step can also identify potential safety issues that have not yet resulted in crashes.
Reviews of crash data can identify prevalent patterns related to time of day, patterns of movement, weather conditions, driver behaviours, and other factors. These reviews can use bar charts, pie charts, or tabular summaries which are useful for displaying descriptive crash statistics and making patterns visible. Crash diagrams are also useful for summarizing the data visually. A crash diagram is a two-dimensional representation of crashes that have occurred at a location.
Diagnosis may also involve a site visit to observe road user behaviours and site conditions not observable from crash and other data records. It is advantageous to conduct site visits under multiple conditions (e.g., day and night, peak and off-peak travel times, dry and wet).
There are also more sophisticated methods that can be used as diagnostic tools to identify if a particular crash type is over-represented at the site compared to other similar locations. The Probability of Specific Crash Types Exceeding Threshold Proportion performance measure is an example. This methodology is described in detail in the Highway Safety Manual.
Potential treatments to address diagnosed causes of crashes can be identified through manuals and other published sources. Relevant information to consider in selecting a treatment include: what crash types benefit from the treatment, what crash type risks may increase from the treatment, what location characteristics are suitable for the treatment, and the treatment costs. Examples of such sources include:
Additional sources of information are listed under What tools are available to aid the diagnosis of sites and selection of countermeasures.
The data required for diagnosis and countermeasure selection includes crash data, traffic volumes, pedestrian and bicycle counts, geometric and traffic control information and other field conditions.
National and local jurisdictional design guidelines related to signing, striping, geometric design, traffic control devices, roadway classifications, work zones, etc. should be used to assess if standards are met at the location under study.
Diagnosis and countermeasure selection requires people with a background in road safety sufficient to understand the causes of crashes with respect to geometric, traffic control and road user behaviour factors. If advanced methodologies are applied, persons with knowledge of statistical analysis should be involved. A general knowledge of roadway design and traffic control is also beneficial when it comes to selecting appropriate countermeasures.
There are several tools available to aid the diagnosis and countermeasure identification steps of the safety management process.
- Highway Safety Manual
This report provides an overview of AASHTO’s Highway Safety Manual (HSM), including the Part B Roadway Safety Management Process that includes a chapter on diagnosis. FHWA Reliability of Safety Management Methods – Diagnosis
The Reliability of Safety Management Methods: Diagnosis information guide describes various methods and the latest tools to support diagnosis. The target audience includes data analysts, project managers, and program managers involved in projects that impact highway safety. The objectives of this information guide are to 1) raise awareness of more reliable methods, and 2) demonstrate through examples the value of more reliable methods in diagnosis. This information guide compares more reliable diagnosis methods to traditional methods which are more susceptible to bias and may result in less effective decisions. Readers will understand the value of and be prepared to select more reliable methods in diagnosis.- FHWA Website on Proven Safety Countermeasures
FHWA’s Proven Safety Countermeasures initiative (PSCi) is a collection of 28 countermeasures and strategies effective in reducing roadway fatalities and serious injuries on our Nation’s highways. Transportation agencies are strongly encouraged to consider widespread implementation of PSCs to accelerate the achievement of local, State, and National safety goals. These strategies are designed for all road users and all kinds of roads—from rural to urban, from high-volume freeways to less traveled two-lane State and county roads, from signalized crossings to horizontal curves, and everything in between. Each countermeasure addresses at least one safety focus area – speed management, intersections, roadway departures, or pedestrians/bicyclists – while others are crosscutting strategies that address multiple safety focus areas. - Crash Modification Factors Clearinghouse
The Crash Modification Factors (CMFs) Clearinghouse provides a searchable database of CMFs along with guidance and resources on using CMFs in road safety practice. - International Road Engineering Safety Countermeasures and their Applications in the Canadian Context
This report is dedicated to finding countermeasures from OECD countries that have the potential to be transferred to Canadian applications.
Term | Definition |
Countermeasures | Interventions applied to reduce crashes, e.g. rumble strips |
Facility | Infrastructure provided for road user movements, e.g. roads, bicycle lanes, sidewalks |
Hotspots | Locations identified as having a high number of crashes compared to other locations |
Rumble strips | Textured strips installed on the road to alert drivers through tactile vibrations if they unintentionally veer off the roadway or across the centerline |
Cable median barrier | A safety barrier installed in the median of a divided highway composed of high-tension cables supported by posts |
High friction surface treatments | Applications of specialized materials or coatings on the road surface in increase friction between vehicle tires and the pavement |
Curve warning signs | Traffic signs used to warn drivers in advance of upcoming curves in the road |
Signal backplates | Panels mounted behind traffic signal heads to enhance the visibility of traffic signals |
Countdown pedestrian signals | Pedestrian crossing signals that display a numerical countdown indicating the time remaining for the pedestrian walk signal |
Educational campaigns | Public awareness initiatives designed to inform, educate and change behaviour related to road safety |
Crash tree diagram | A visual representation or chart that illustrates the frequency of crashes by crash types and other involved factors |
Road diets | A reallocation of road space by reducing the number of through lanes and adding a two-way left-turn lane, often with the addition of bicycle lanes or other facility |
Optical speed bars | Visual speed indicators painted on the road in the form of bars that provide optical cues to encourage motorists to reduce their speed |
Speed tables | A traffic calming device similar to a speed bump but that is longer and with a flat top |
Centreline hardening | A form of traffic calming that reduces the turning radius for vehicles using physical measures on the roadway to encourage slower speeds |
Gateway treatments | Physical measures taken where a rural road meets a more urban area to increase driver awareness that posted speed limits are changing, such as landscaping, signage or road markings |
