17.10 Speeding Countermeasures: Setting Appropriate Speed Limits

  • When establishing speed limits, account for a road’s unique traffic patterns, design features, and environmental characteristics. Table 17.2 lists strategies and guidelines for establishing speed limits. If speed limits do not align with motorist expectations, they are more likely to be ignored.
Table 17.2 Recommendations for Establishing Speed Limits
Strategy Guideline
Establish context-appropriate speed limits
  • Consider:
    • Design speed of a major portion of the road
    • Vehicle operating speeds (based on a range of 85th percentile speeds measured using spot speed surveys)
    • Road safety experience (based on crash frequencies and outcomes)
    • Enforcement tendencies (level of law enforcement, law enforcement willingness to let vehicles drive over the speed limit
    • Implement Variable Speed Limits
  • Useful for coping with:
    • Predicable events (e.g., construction activity, school hours)
    • Unpredictable events (e.g., traffic incidents, poor visibility from inclement weather)
    • Differential Speed Limits for Heavy Vehicles in High-Speed Areas
    • Posting a lower speed limit for heavy trucks in high-speed areas can help reduce the severity of collisions that involve trucks.

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