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Variable Speed Limits: The New Normal

In early June 2016, BC Transportation Minister Todd Stone announced the installation of variable speed limit signage systems [VSLs] on three BC highways as part of a pilot project to determine their effectiveness in reducing crashes in which inappropriate speed for weather conditions is a significant factor.

At a cost of $12.5 million, the signs are now in place on the Coquihalla, the Sea-to-Sky, and the Trans-Canada near Revelstoke.  While it is elementary for all licensed BC drivers to know that the rectangular black on white speed limit signs are setting the ‘maximum’ operating speed ‘in good conditions’, and the rectangular black on yellow speed limit signs are recommending speeds for conditions ahead such as a curve or an exit ramp, statistics indicate that adjusting down to the safest speed when the conditions are ‘not good’ is problematic.

US statistics, for example, show that between 2000 and 2009, 12.6 percent of fatal crashes occurred on wet pavement.  Vancouver drivers, of course, hardly need carefully calibrated numbers to persuade them of this problem.  Once it starts raining, fender-bender announcements immediately flood our traffic radio airways.

In a 2012 report from the US Federal Highway Administration, Guidelines for the Use of Variable Speed Limit Systems in Wet Weather the explanation for the effect of adverse weather events on traffic flow reads as follows: “…driver behavior varies during adverse weather events based on experience and risk tolerance.  Therefore, some drivers may slow down when they encounter wet weather, but others may maintain or even increase their speeds.

The result of varying behavior may be a significant speed differential between slower and faster vehicles on the roadway.  It is generally accepted that an increase in speed differential increases the potential for conflicts between vehicles, which in turn increases the potential for crashes.”  For this reason, “VSL systems based on weather conditions can work to reduce the speed differential under unpredictable events and encourage more uniform speeds.”

VSLs are not ‘a whole new thing’ for BC nor for other North American highway systems.  For decades, they have operated in school zones, work zones, and in areas where even in ‘good conditions’ the sight distance is less than the stopping distance—think tightly curving roads on scenic highways, for example.

What is new about ‘weather condition VSLs’ is the linkage between higher than average crash rates in areas with a history of adverse weather conditions where safe speed adjustments measure in significant increments—say, roughly of 15 kmh.  For the BC study, a propensity for traffic congestion plugs to form may also have been a factor in determining the chosen locations. Technological innovation has also enabled the ‘real-time’ functionality that is a key component of weather condition VSLs. Modern VSL systems include “sensors, communications, power, operator confirmation and override, and a police notification process.”

Calculation of the appropriate speed for the weather conditions, display design, the zone parameters, and signage location—all are important components.  And of critical importance for “both driver safety and driver trust in the system’s accuracy” are effective operation and ongoing maintenance plans.

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