Does speed kill? Or, at the very least, does speeding on our streets lead to more accidents and injuries?

Yes. And no.

Understanding just how speed affects traffic and traffic accident rates is critical to Edmonton’s ongoing debate about the use of photo radar, especially given that the city will not rule out hidden, automated photo radar cameras.

According to Hugh McGee, a Virginia-based transportation safety engineer, it’s unwise to draw conclusions from “simple comparison studies” – ones that analyze accident rates at the same location, first without photo radar, then with it.

“Too many other factors contribute to accident risk,” such as weather, time of day, visibility, the condition of the road surface, the condition of the roadbed, road design (blind corners kill, too), inclines and others.

A road slick from rain, snow or ice is as likely to cause serious accidents as a driver going nearly 20 km/h over the speed limit on a dry, straight road. Poor signal light location has an impact, as does badly painted lines.

Even plenty of potholes and cracks can raise the risk of accidents.

So claiming (as the city of Edmonton loves to do) that “every one-km/h increase in speed leads to a three-per-cent rise in collisions,” is, according to McGee, “simplistic.”

Believe it or not, setting a speed limit too slow for a given location also adds considerable risk.

According to the Institute of Transportation Engineers, the best speed limits are those set at the 85th percentile. That’s the speed up to which 85 per cent of motorists drive even when there are no speed limit signs.

Limits set too far below that cause accidents because some motorists will be puttering along at the posted limit or less, while others are driving at a higher speed that feels more natural. The mix of vehicles at various speeds is risky.

So unless the city knows what the 85th-percentile speed is on the streets in town, and posts its limits accordingly, it could actually beincreasing accident risk by keeping limits unnaturally low.

A recent B.C. study conducted using speed-limit enforcement stats combined with insurance claims found that drivers going more than 30 per centunder the posted limit were far more likely to cause accidents than those going as much as 15 per cent above.

On a highway where the speed limit is 100 km/h, that means someone driving 115 km/h is a much smaller risk to his fellow motorists than someone doing 70. On a street where 60 km/h is the limit, a driver going 40 is more dangerous than one going 70.

Yet you never hear our city’s administration announcing a crackdown on slow or indecisive drivers.

Increasingly, too, there are studies that show that photo radar has little effect on driving speeds. Motorists come to view photo radar as a tax they have to pay when they’re in a hurry.

The surest way to make streets safer is to focus on the two to five per cent of drivers who are going 20 per cent or more over the speed limit. They create the most havoc.

But in Edmonton, that would require the city government to give up tens of millions of dollars in revenue from its useless photo radar machines.

Thankfully the proposal to have the city install suitcase-sized photo radar boxes along our streets (in addition to all the pick-ups with photo radar in them) is a low priority for the office of traffic safety.

But given that council and the administration are obsessed with the one-dimensional idea that speed kills (and are heavily dependent on money from photo radar), the idea could quickly climb higher.

lorne.gunter@sunmedia.ca