An explainer on the Ontario government’s move to review highway speed limits
TORONTO — Ontario’s government plans to launch a review of speed limits on provincial highways, with the transportation minister musing that some of those roads can safely handle traffic at 120 kilometres per hour. Here’s a look at the context around the issue:
HOW ARE SPEED LIMITS DETERMINED?
According to Mohamed Hussein, a transportation engineering professor at McMaster University in Hamilton, there are effectively three speed limits for any highway: the “design” speed, which is the maximum speed at which a road can be safely navigated and determines every element of its design; the “operating” speed, which is how fast traffic actually moves on the road; and the “posted” speed, which is displayed on roadside signs. The posted speed is a product of many factors, including a commonly used measure called the 85th percentile: the speed at which 85 per cent of traffic will travel at or below. Political considerations are also a factor, such as when the United States passed a national speed limit law in the early 1970s. The legislation was enacted in an effort to conserve fuel amid an embargo on oil shipments from the Organization of Petroleum Exporting Countries (OPEC).


