Traffic & Safety Study Methodology

The City of Durham uses a combination of methods to identify study locations. Some common techniques used are:
  • Accident rate method
  • Cost method
  • Frequency method
  • Frequency-rate method
  • Hazard index method
  • Rate quality method
  • Severity method
  • Statistical method
Experts agree that there is not 1 method used that is superior. The city uses a systematic approach based on the frequency and severity methods to identify candidate study locations. However, as needed, other methods may be used as supplemental tools.

Accident History
A comprehensive 8-year accident history is maintained for all intersections, with 5 or more recorded accidents. From the compilation a ranking, based on average crashes per year, is used to find problem locations. Simultaneously, an annual ranking of intersections based on severity is compiled. In combination, this can be an effective means to hone in on locations that have both a high frequency of accidents and the highest likelihood to result in severe injuries.

From here an in-depth engineering study is undertaken to determine the cause of the accidents. Possible countermeasures are selected based on a field investigation, collision diagrams, other transportation engineering studies, technical literature, and previous experience.

Countermeasure Selection
The selection of a countermeasure is crucial and typically is discussed by a group of engineering professionals internally, before a final decision is made. After a countermeasure is selected, the recommendation is forwarded to the governing agency. Upon approval the countermeasure is implemented. For evaluation purposes a "before condition" diagram is sealed on the date of implementation. An "after condition" diagram will be opened and maintained for a period equivalent to the time period used to identify the problem, typically 3 years. The "before" and "after" condition is evaluated on a regular basis to monitor the effectiveness of the improvements.

Common Probable Causes
Common probable causes include, but are not limited to the following:
  • Absence of a left-turn traffic signal phase
  • Improper pavement markings
  • Inadequate advance warning signs
  • Inadequate roadway lighting
  • Inadequate signal timing
  • Large turn volume
  • Restricted sight distance
Typical Accident Reduction Countermeasures
Typical countermeasures include, but are not limited to the following:
  • Delineators
  • Guide signs
  • Modified clearance intervals
  • Modified signal timing
  • Pavement markings
  • Phasing upgrades
  • Regulatory signs
  • Removal of sight obstructions
  • Sign placement
  • Signal back plates
  • Signal flashers
  • Signal visors
  • Supplemental signal heads
  • Traffic signals
  • Warning signs
We encourage the public to report safety problems. If you see a safety problem, contact Larry McGlothlin by email or at 919-560-4366, ext. 36435.