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Planning Guidance for the Installation and Use of Technology Devices for Transportation Operations and Maintenance

ENTERPRISE Transportation Pooled Fund Study

VSL Guideline 3 - Safe Travel Speeds for Conditions

Purpose: To maintain safe travel speeds during periods when road and/or driving conditions may be impacted.

Engineering judgment should be used to assess potential implications that may result from installation of VSL. This planning guidance does not mandate the use of VSL.

1. The area in consideration is a segment or corridor (typically a minimum of 2 miles or longer) and not an isolated spot location (such as a bridge or a vertical curve) where other technologies may be more appropriate.

AND

2a. The area regularly experiences adverse conditions (e.g. snow, water on the road, fog, wind, blowing snow, animal migrations) that result in traffic congestion, slow downs, low visibility or safety hazards for travelers.

OR

2b.The area experiences rare conditions that result in traffic issues, but the resulting traffic issues are known to impact large numbers of vehicles (e.g. locations with crashes that occur every 1-2 years but results in incidents, impacting numerous vehicles).

OR

2c. The geography or geometry of the area pose a known risk for traffic issues when traveled by vehicles at varying travel speeds.

AND

3. The critical crash rate for the segment or corridor is higher than expected for similar segments within the state, based upon the judgment of local engineers. For example: is the stretch of road a known location for high crash rates.

AND

4. The area experiences regular speed differentials of at least 10 mph of travel speeds below bosted speeds between drivers that are believed to contribute to crashes. (Speed differentials may be due to vehicles traveling slow for various reasons (e.g. commercial vehicles with chains limiting speed, cautious or unfamiliar drivers traveling considerably slower than other drivers), weather conditions that may not be obvious to all travelers (black ice, freezing conditions, water on the roadway, fog, heavy winds, blowing dust), or other factors). (1)

Reference:

1. MUTCD. 2009.