On the A16 Motorway in the Netherlands, an automatic fog warning system prompted drivers to slow down by 8 to 10 km/hr and drive at more uniform speeds; however, during extremely foggy conditions the system increased the average vehicle speed by 31 km/hr matching the recommended speed.
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A16 Motorway

Evaluation of A16 Motorway Fog-Signaling System with Respect to Driving Behavior

Summary Information

On the A16 Motorway in the Netherlands, an automatic fog-signaling system was implemented October 1991 to elicit safer driving behavior during fog. The system uses 20 sensors along the 12 km stretch to measure visibility. Based upon the visibility distance calculated, a certain speed limit would be shown on overhead message signs. For a visibility distance greater than 140 m, no speed limit was shown. For those from 70 to 140 m, the 80 kph speed limit was posted, and for those less than 70 m, the 60 kph speed limit was displayed.

The system has a positive effect on speed choice in fog: it was found to result in an additional decrease of speed of about 8 to 10 kph and a slight reduction in standard deviation of the speed. In extremely low visibility (< 35 m), the system had an adverse effect. The average speed with the system in this situation was 60 kph, versus 29 kph without. Using the relation between mean speed and number of accidents, a reduction of 5 kph would result in a similar reduction in the number of accidents by approximately 15 percent. The system showed small or no effects in other measures of driving behavior such as following distance, time headway, and time to collision.

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