3 Speed Management
3.2.9 Fog Signaling System – A16 Motorway - Netherlands
The information on the Fog Signaling System was obtained from the report Examples of Variable Speed Limit Applications(6 ), and from the FHWA ITS Joint Program Office/Mitretek ITS Benefits Database.(7 )
Impetus for the Activity
The impetus for the deployment of the fog signaling system was to elicit safer driver behavior during foggy conditions.
Description of the System
The system was implemented on the A16 Motorway near Breda in October 1991, and is currently active. The system covers 12 km with VSL signs every 0.6 to 0.8 km. The system obtains its visibility distance inputs from 20 visibility sensors, and has automatic incident detection capability.
The posted speed is 100 km/h. When visibility drops below 140 m, the speed limit is dropped to 80 km/h. If visibility drops below 70 m, the speed limit is dropped to 60 km/h. If an incident is detected, the speed limit is dropped to 50 km/h on the first sign upstream of the incident, and 70 km/h on the second sign upstream.
System Performance
No problems with system performance were reported.
System Effectiveness Evaluation
The effectiveness of the system has been formally evaluated. The measures of effectiveness included speed, standard deviation in speed, following distance, time headway, and time to collision.
Following installation of the system, drivers reduced their mean speeds by approximately 8 to 10 km/h during foggy conditions. There was also a slight reduction in the standard deviation of the speed. In very low visibility conditions, the system had an adverse affect, and actually caused drivers to go faster than they traveled without the system. There were small or no effects shown on the other measures.
No information was provided on the effect on the frequency or severity of crashes.