3 Speed Management
3.2.11 Target Crash Population
General
The population of crashes that is targeted by general VSL systems, such as the New Jersey Turnpike system or the A2 system in the Netherlands, would be any crash in which the driver was driving too fast for conditions or in excess of the posted speed limit. From a FARS query(2), it was determined that 6846 rural fatal crashes occurred in 1998 that had driving too fast for conditions or in excess of the posted maximum as a driver-related factor. This represents almost 32 percent of the rural fatal crashes (with known road class) that occurred in 1998.
Such systems should have the maximum potential payoff in areas of relatively high traffic or frequent unpredictable weather conditions, where a number of factors can influence the speed at which drivers should be traveling anywhere along a long stretch of roadway. Coupled with these systems, however, would need to be an effective means of convincing drivers to adjust their driving according to the warnings or advisories given. One potential means of doing this may be through enforcement, either conventional or automated.
Winter Weather and Winter Road Conditions
VSL systems such as those in Washington, Arizona, and Finland target the population of crashes that occur under winter weather conditions (snow, sleet, and combined sleet and fog) and under winter pavement surface conditions (snow, slush, ice).
Using a FARS query(2), Table 2 was developed. It can be determined from the table that approximately 2.3 percent or 489 of the rural fatal crashes in 1998 occurred under winter atmospheric conditions (sleet, snow, and combined sleet and fog). Using a different FARS query, it can be determined from Table 3 that approximately 3.6 percent or 772 rural fatal crashes in 1998 occurred under adverse winter pavement surface conditions (snow, slush, ice).
While the potential treatable crash population is not overly large, these systems should have the greatest potential payoff in areas where winter conditions are severe, frequent, unpredictable, and involve non-local drivers.
 Table 2: Manner of collision by atmosphere condition - rural fatal crashes - 1998. |
 Table 3: Manner of collision by pavement surface condition - 1998 rural fatal crashes. |
Fog
The VSL systems deployed in Australia, Tennessee, Nevada, and the Netherlands (A16) target the population of crashes that occur in fog. From Table 2, it can be determined that approximately 2.2 percent or 472 of the 1998 rural fatal crashes for which atmospheric condition and road class were known, occurred under foggy conditions. Again, while the potential treatable crash population is not overly large, these systems should have the greatest potential payoff in areas where fog conditions are severe, frequent, and unpredictable.