Highway Safety & Design

Surveyor AheadSome 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.

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Comments

Response via E-Mail

This is a very good subject and one that obviously is not understood by the majority of drivers, including semi drivers.

If one were to just dwell for a moment on ‘total stopping distance’ and the factors therein, the answer is quite simple:

Perception Distance: the distance travelled while a driver assesses the situation ahead – increase the speed the distance becomes longer for the same amount of time passed.

Reaction Distance: the distance travelled while a driver is moving a foot from the accelerator pedal to the brake pedal (should a stop be required) – again increase the speed the distance increases as well.

Braking Distance: Now here is the big one! Physics tells us that if you double the weight of the vehicle and you want to stop the vehicle in the same distance the power must be increased two fold. If the speed were doubled, the stopping power must be increased 4 times. Therefore any increase in speed has a dramatic affect on a person’s ability to stop a motor vehicle.

Let’s consider some of the factors involved in braking distance:

Vehicle condition – this includes brakes, tires, and the entire braking system of the vehicle. With today’s living costs many people, unfortunately, let proper vehicle maintenance slip by as the cash is not there to keep the vehicle is excellent mechanical condition.

Weight and Speed – how often do we see overloaded vehicles on the highway, especially during holiday season, speeding to get to a destination?

Road condition – even the best of tires on a rain slicked road after a long dry spell is not enough to prevent skids when the vehicle is travelling too fast or the cruise control is ‘on’ when the driving wheels go through a deep puddle of water on the road and loose traction.

Road slope – any vehicle can gain speed when going downhill even with the engine turned off – just imagine the extra force created with the engine on in a downhill situation.

Unfortunately, too many drivers consider a driver’s licence a right and not a privilege. Because of this mentality it appears that so many of them figure ‘they own the road’ and therefore can do what they please without any respect for other motorists or road users.

Contributed by E-Mail

As a Civil Engineering Technologist (Retired), I had the priviledge of working on several sections of the Island Highway with a very talented Highway Design team. Like creating a good movie, designing a good road system is both Art and Science, and as stated in your article, roads (and movies) are constrained by terrain and budget. Some watching a movie, let it wash over them and enjoy the experience, while others (like myself) study it and ask "how did they get that shot?" Many drive our roads without any notice to there arrangment, but one only has to drive south to Washington State, even on I5, to notice a difference in the "feel" of the road.

Let's consider terrain first. A road will have to change grade and direction in concert with the ground. Most of the Island Highway's grades are in the range of 6-8%, meaning a change in elevation of up or down 6m over the distance of 100m. Doesn't sound like alot but 6% will slow a loaded truck or happy family towing a travel trailer, hopefully in the right lane. To change the grade designers uses a vertical curve. The higher the design speed, the longer the curve has to be, for comfort and visibility. A crest designed with the minimum length will allow a driver time to see and react to an object in the road about one foot high (say an injured person lying down), that's the "science", but that design "feels" unconfortable to the driver who really wants to see farther over the crest. So a good designer will use a longer curve than "code" if other factors allow, for a more comfortable drive, that's the "art".

The reason for the reduced speed of 90k/hr at the various intersections on the Island Highway has more to do with drivers reaction time, than the road itself. There will be much more to react to, and at a much higher closing speed, at an intersection. The reduced speed allows the driver more time (distance) to react. At 110k/hr you are travelling over 30m in one second, at 90k/hr it is reduced to 25m per second. Ask yourself if you would rather have a vehicle stop 3m in front of your spouse, child, friend or pet versus 3m PAST, with unthinkable consequences?

Prior to the completion of the Island Highway, there were few places to legally drive over 90k/hr on the island, like the road to Cowichan Lake. The "new" Island Highway is an amazing engineering project and a pleasure to drive as well as a priviledge to drive at higher posted speeds. I hope this adds to the explanation of the question about the reduced speeds at intersections posed by the reader, and please enjoy this highway responsibly as it might be my family at the intersection.

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