Sensors + Intelligence + Enforcement = Compliance for Variable Speed Limit Signs
Imagine you’re driving a road that you know is prone to accidents due to weather conditions or sudden congestion. It could be a winding highway on the way to your mountain cabin, or it could be that expressway that runs through the city. There have always been sensors in the road to count traffic and monitor speeds, but now there are highways where sensors detect rain, snow, temperature, loss of traction, low visibility, and wind. This is sent to a central server that digests this information and recommends a lower speed to the sign in advance of the condition. So, if there’s an icy patch in the fog around that corner, you should get a warning to reduce speed.
And that’s where the intelligence lies. Information from the sensors is a non-stop fluctuating barrage of variables streamed to a database and interpreted by a program. Is it both cold and icy? Is there both low visibility and snow on the ground? Is the traffic already reducing speed? What should the algorithm recommend as a speed? What is the fastest safe speed to drive in those conditions and what threshold needs to be reached to drop the speed and for how long? An algorithm could change the speed every 1/1000th of a second to any number above 0 and below the posted speed limit, but while that might be accurate it is not functional.
Intelligent people can step in at this point. A traffic engineer can set thresholds and buffer times and filters so the progression of the signs makes sense, such as it can only step down increments of 10 every 60 seconds, and if it steps down more than 30 it has to step down the previous sign to smooth out the speed. Or the signs could make recommendations and a human operator would have to approve the speed change. There would be a trained operator at the controls verifying road conditions on available cameras and making speed adjustments as necessary.
And then there’s enforcement. Some Variable Speed Limit (VSL) systems have failed because the laws were not in place to enforce variable speed signs. A driver could refute the speed and demand proof that the sign had changed when the police were down the road and unable to verify it themselves visually. A database reporting system makes it possible to give tickets that will stand up in court; at precisely this minute the road conditions or the traffic congestion were X, and the sign was changed to X at X o’clock, and the driver was travelling X at X o’clock. It’s all in the database and it’s all viewable online for everyone: the police, the judge, and the driver. Without this secure verification process, there can be no enforcement and therefore less compliance– and little benefit to the drivers.
VSL signs are also deployed lane-by-lane in urban settings, where the signs might include a red x indicating construction or obstacles in the lane ahead. In busy city highways the goal of the city is to reduce collisions, and the systems are effective at doing that. An added benefit is drivers can see how speeds tend to not reduce in High Occupancy Vehicle lanes, and perhaps that will change future driver behaviour.
“Based on collected data, it is observed drivers are reducing their speeds on variable speed corridors when a lowered speed limit is posted,” said Jennifer Locke (Project Manager, BC Transportation and Infrastructure) to the Transport Association of Canada
IBI has experienced consultants who understand traffic engineering, traveler information services, data warehousing, data visualization, sensor deployment, smart cities, and ITS strategies. Our deployment of three VSL corridors in BC has won four recent engineering and road safety awards. IBI Group worked closely with the BC Ministry of Transportation and Infrastructure to design the software that manages these (and future) VSL corridors.