Believe it or not, there is science involved in setting speed limits.
A lengthy missive from a reader, more accurately described as a rant, raises the issue of speed limits.
This person, who shall remain nameless as he is likely going to end up in court, questions why and where radar traps are located. He goes on to question everything from the training police officers receive to the reason various agencies and outlets report 80-120 km/h acceleration times. But the central issue is that he was ticketed for exceeding the speed limit while passing a slow truck over the crest of a hill.
A day after receiving this communication, I was driving through a familiar part of my city and encountered a roadside radar setup. In this area, there is a straight four-lane road with excellent visibility, that ends with a curve where the speed limit drops from 80 to 50 km/h. I've heard many people complain about the frequent enforcement of this change in speed limit - most of them refer to it as a "speed trap" or "revenue generator."
I have not been stopped, warned or ticketed at this location. As a regular, I have come to expect police to be there. Upon reflection, and after a discussion with officers familiar with the location, I have a different take on the situation. In the past two years, there have been four very serious multi-vehicle crashes at this location - two resulted in deaths and all resulted in injury to two or more people.
There are a number of reasons for the change in speed limit, including:
- The fact you are leaving a rural area and entering a populated one.
- A grocery outlet is located adjacent to the road at this point and people are pulling out from that parking lot onto a road with limited visibility.
- A large school is also in the area.
Is it a speed "trap?" Yes. Because motorists regularly fail to slow much below 70-80 at this point. Is it a revenue generator? Yes. Because so many motorists fail to slow at this point. Are police manning the area to generate revenue? I seriously doubt it. They are responding to residents and safety officials who want to send a message - slow down! Revenue results.
Believe it or not, there is science involved in setting speed limits. Generally speaking, speed limits are established based on issues such as traffic flow, sight lines and other measurable factors. The common belief is that speeds limits should be set in consideration of road characteristics and the 85th percentile speed of those using that road.
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In a paper presented at the 20th annual Canadian Multidisciplinary Road Safety Conference in June, a quartet of engineers from CIMA, a Canadian engineering and consulting firm, stated that the 85th percentile speed "gives a good indication of the speed suggested by the roadway." But it went on to say that unless engineering modifications are made or education and enforcement are used to reduce operating speeds, "reducing a speed limit by simply changing signs will most likely not alter drivers' behaviour."
In another paper presented at the same conference, representatives of the Department of Civil Engineering and Western Transportation Institute at Montana State University reported on a study of how drivers determine speed at locations with restricted views, specifically around horizontal curves. They studied seven sites on a 16-kilometre stretch of well-travelled mountainous roads. Two with clear sight lines, two where the view was restricted by a corner and three where the view was restricted by both a turn and distance. Their findings:
- In the absence of effective enforcement, drivers tend to travel at speeds they perceive as safe, regardless of the posted or advisory speed limits.
- The vast majority of observed speeds were notably higher than the maximum safe speeds found using "alignment and sight distance design equations."
The bottom line is that without enforcement, drivers are likely to exceed posted limits. The argument of course is that those limits are too low. But local extenuating circumstances are often a factor.
Halifax-based Richard Russell runs a driving school.
globedrive@globeandmail.com
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