Some time ago I wrote to the engineer in charge about the speed limit of 90 km/h at intersections with traffic signals on the Inland Island Highway. I requested an explanation of the limit at these intersections and also why the 90 km/h limit was placed so far away from the intersection. Not only do most motorists ignore the limit at the intersection, I would guess that 100% ignore the limit where the signs are posted. The police have advised me that it is necessary to slow to 90 km/h at the sign although there seems to be no logical reason to do so.

I have often thought that it must be a real challenge to be responsible for highway design here in the province of B.C.. Not only do you have mountains and rivers to span, hopefully within budget, you have to contend with the behaviour of the people who drive on them. In examining my own perceptions I often think that some things are not logical too.

Most often, if I try hard enough I can find out the answers but sometimes not. However, it can be dangerous to disregard something that has been carefully planned by professionals based only on your own assessment of logic.

The short answer I have received in the past is something along the lines of "design practices call for it to be done that way." I imagine that is the polite way of saying that they probably couldn't distill a university degree and years of experience into a ten minute conversation in a way that I would understand enough to see the point. Unless research shows a better way, this is probably the safest approach because we know the outcome.

One example from my collision analysis training might be a vertical view obstruction. The highway looks nice and straight, but a dip in the road can hide a small car entering or exiting at a side road or driveway completely. The posted speed might be 70 km/h even though 90 looks fine, but 70 is needed in order to perceive and come to a stop in time if something should be hidden in that dip. This might not seem logical until you understand what is involved.

I can't provide a definitive answer to your question on this particular situation, but I would encourage you to learn more on your own. The internet can be a wealth of information, both in web publications and the ability to communicate with knowledgeable people who will take the time to answer. Don't be discouraged because this engineer failed to answer, rather consider it a challenge.