The Speed Limit on Whitemud Drive, It’s Not As Easy As Just Changing The Signs

The topic of changing the speed limit on Whitemud Drive from the currently posted limit of 80km/hr to 90 or even 100km/hr was something that first came to my attention on Saturday. My reaction, via Twitter, was fairly blunt:

In light of Mayor Mandel’s comments this afternoon referring to the Transportation Department as a “dictator,” I thought it might be worthwhile to expand a little on my thoughts from Saturday.

The issue, for those who don’t know, is that the mayor would like to see the speed limit on Whitemud Drive raised, and based on a report which was prepared for Council, the Transportation Department is saying that the current limit of 80km/hr is appropriate. I’ve neither seen the report, nor had anything to do with any projects on the Whitemud so I have no specific information, but I’ve got a couple of ideas about what the problem might be.

The first is consistency. Ideally, the posted speed limit along a roadway wouldn’t change as you drive from one end to the other. A consistent speed limit provides the driver with a certain expectation, and avoids situations where a driver misses a speed limit change creating a potentially unsafe situation by travelling to fast or too slow for a particular section of road.

Speed limits are usually based on the characteristics of the roadway, including the horizontal and vertical geometry, lane widths, and intersection or interchange spacing. The problem with a consistent speed limit is that you end up posting the speed based on the lowest design segment of the roadway. In the case of the Whitemud, if I had to guess, I would say this is the section around Terwillegar Drive. where there are two relatively closely spaced interchanges, the interchange at Terwillegar Drive has a non-standard design with left-hand entrances/exits, and there is a 90 degree curve in the roadway alignment.

I don’t know for sure, but I suspect that other sections of Whitemud Drive, if looked at in isolation, would be able to safely accommodate a posted speed higher than 80km/hr. And that’s the rub, because a speed limit should not only be consistent but also credible. If the east and west ends could be safely driven, based on the chosen criteria, at say 100km/hr, then a choice needs to be made between consistent and credible. So, in a way both the mayor and the Transportation Department are sort of right in this situation.

But there is a second issue that might be playing a factor here, and that is the actual travel speed. I know I’ve heard the rumour that anything the police use to measure your speed has limited accuracy, and that you can’t get a ticket if you’re not going more than 10km/hr over the speed limit. I’ve never gotten a speeding ticket so this might be true, but it seems suspect to me.

That people speed isn’t automatically a problem though since most roads are not designed to be driven at the speed they are posted at; often times the difference is 10km/hr. Think of it as a factor of safety for the driver. But if people believe they are “allowed” to drive 10km/hr over the speed limit and the travel speeds are already approaching the safe design limit, then increasing the posted speed might result in an increased number of collisions. And I think we can all agree that would be undesirable.

So what do you do? You could raise the posted limit and increase enforcement, but I doubt that would be very popular even if it’s done in the name of safety. Or just leave it the way it is, which is also going to be unpopular. I don’t know the answer, but I’d be willing to bet this isn’t the last we’re going to hear about it.

One thought on “The Speed Limit on Whitemud Drive, It’s Not As Easy As Just Changing The Signs

  1. Shawn Benbow

    I think Edmonton as a whole has a problem with ‘Credible’ speed limits, and I think with that you’ve nailed it on the head. Most of the Whitemud is likely designed for speeds much greater than 80km/h. That’s why people drive faster than 80km/h. If it were designed for 80km/h speeds, the average speed on that roadway would very likely be much closer to that speed.

    So maybe the solution is to keep the Terwillegar area at 80km/h, and post the rest at 100km/h. Even today people slow down to make the sharp 90-degree bend at 80km/h (there’s a posted speed advisory for 55km/h). I can’t imagine that would change if the speed limit on the rest of the road went up. There is a lot of research (and designs) completed recently that show people drive similar speeds to the design speed, regardless of the posted speed. That’s why people drive 130km/h on Highway 2 — that’s what it’s designed for.

    However, back to the Whitemud. From Anthony Henday Drive (west) to AHD (east), the Whitemud is about 25km long. I understand the argument for consistency, but does it really make sense to under-post 25km of roadway when less than 1 kilometre of the roadway is under designed?

    We don’t post rural highways at 50km/h because once in a while they go through a town. We post them at 100km/h and drop the speed when required due to road and development factors. So when you’re driving on, say Highway 15, the speed limit is 100km/h — but when you reach Mundare, the speed limit drops to 50km/h. We accept this inconsistency, and I’d assume most people would think it would be silly to post all of Highway 15 at 50km/h just to keep the speed limit consistent along the entire corridor.

    We can’t continually reinforce that Edmonton’s drivers are terrible. Post the speeds at a reasonable limit. Force Edmonton’s drivers to actually get better, rather than catering to the lowest common denominator. Look at any other North American city with more than 1 million people. A limited access freeway like the Whitemud would be posted near 100km/h (55-65mph in the U.S.) almost anywhere else.

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