Why is the Highway Designed That Way?

Surveyor Ahead Warning SignDo you ever wonder why some aspect of the highway that you are driving on has been designed that way?  It starts with the Transportation Association of Canada's Geometric Design Guide for Canadian Roads, is supplemented by the Geometric Design Guidelines for B.C. Roads, may require compromise due to local conditions and ends up being what you see through your windshield.

I Have a Question for You

I received the following observations from a reader of my DriveSmartBC newsletter:

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.

I never received an answer.

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.

Constructing Any Road is a Challenge

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.

Personal Perceptions Are Not Always Reliable

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.

Sometimes the Issue is not Obvious

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.

Consider Learning to be a Challenge

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.


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.

As a Civil Engineering Technologist (Retired), I had the priviledge of working on several sections of the Inland 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 their arrangment, but one only has to drive south to Washington State, even on I-5, 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 a lot 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 90 km/h at the various intersections on the Inland 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 110 km/h you are travelling over 30m in one second, at 90 km/h 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 90 km/h on the island, like the road to Cowichan Lake.

The "new" Inland 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.

I would guess that 100% ignore the limit where the signs are posted.

I never received an answer.

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.

Actually, most drivers will react to both a pending speed limit reduction, and of course a pending intersection. Especially on Hwy 19, which most of the time is one of our fastest roadways in BC. I've zipped up and down there many times! Do drivers ignore speed limits? Yes, many do. Others are insistent on obeying them. But that doesn't mean that because someone 'believes' that the majority ignore the limit, that's how all others behave.

Most good drivers have the sense to realize that lowered speeds provide much shorter stopping distance, as well as more time to react to what's transpiring ahead and around them. This is not rocket science ... and your visual attention should be sufficient to identify what's about to happen at the intersection ahead, due to the behaviour of pending cross traffic activating the light. This will result in less braking and fuel waste too.

My Civil Engineering training in Highway Design is now some 50 years old, but the basic principles of physics still apply.

In driving through the Cook Creek area - perhaps one of the areas you are referencing - the signage is telling you to slow down from the posted 110 to 90 because you are going into a downhill grade and may be encountering a signalled Stop which may require heavy braking to come to a controlled stop in the time/distance available.

Similarly, cautioned speeds for upcoming curves/bends in roadways are based on physics for an “average” vehicle - transport  trucks or high centre of gravity vehicles may lose control, while a sports car driver loves (can handle) the physics so to speak.

In many BC locations roads built on hillsides have curves that sink over time and change the design elevation of the curve that can throw a vehicle in the wrong direction; the Fraser Canyon has quite a few, and I know a curve on Hwy 37 that has well over foot of asphalt buildup to overcome the elevation changes as the highway continually slips downhill.

To summarize, my belief is that properly engineered highway design is all about safety, but design is continually challenged by drivers feeling that their vehicles can easily handle speeding beyond the posted speed or weather condition; you had a career of witnessing many who were wrong in their decision.

As with highway maintenance the lowest bidder is accepted, and it has now reached the point the contractors lacks the resources to meet contract conditions. Excuses are made for the failures.

And it is the same with new construction. An RFP goes out and even though the engineers may have recommended one alignment if the contractor(s) come back with a different recommendation it is normally accepted.

Some jobs have a warranty period but I know of one situation that no repairs were made till the last year. Maybe if they had been repaired when the problems first appeared in six months by the end of the warranty period it would have been fixed.

This has nothing to do with speed limits but it does have a bearing on sight lines. The construction is done to optimise the contractors profit and probably just meeting minimum requirements of the contract. Often within six months settling and sloughing occurs that the minimum conditions are no longer met.

So when you see the curves and you wonder why didn't they go straight or you go up a hill just to go back down again and your bouncing around because the pavement is not smooth, that just maybe the reason. Profit.