Speed and following distance

What a speed limit is telling you

A speed sign gives the highest legal speed for that road in good conditions: dry surface, clear view, light traffic, a vehicle in sound order and a driver who is awake. Take any of those away and the safe speed drops below the number on the sign, while the number itself stays where it is. Every province writes this into its law in some form, usually as a duty to drive at a speed that is reasonable for the conditions, which is why a driver can be charged for driving too fast for a snowstorm while still under the posted maximum.

Limits themselves are provincial. The default speed for a road with no sign on it, the maximum on rural highways, and the special limits for school zones, playground zones and construction areas are set by each province and sometimes by the local municipality, so look them up for the place you are being tested in rather than carrying a number across a border. Speeding penalties in construction zones and school zones are heavier than elsewhere in most of the country.

On some curves and ramps you will see a smaller yellow tab under a warning sign showing a lower speed. That is an advisory speed for that feature, and it assumes good conditions too. Slow to it before you enter the curve, not while you are in it, because braking mid corner is where the grip runs out.

Why speed changes everything about stopping

Stopping is three things added together. First you have to notice the hazard. Then you have to react, which takes a moment while your foot moves. Only after that does the vehicle start to slow, and it keeps moving the whole time. Perception and reaction distance grow in a straight line with speed, but braking distance grows much faster than that, so a modest rise in speed makes a large difference to where you finally stop.

This is why the sensible way to think about speed is not the number on the sign but the distance to whatever you might have to stop for. Drive so that you can stop within the distance you can see to be clear, on your side of the road. Over a crest, around a bend, in fog and after dark, that distance shrinks, and your speed has to shrink with it.

The same physics explains why a heavier or loaded vehicle, a vehicle towing a trailer and a vehicle with worn tires all need more room. It also explains why the last part of a stop feels easy while the first part is where the damage happens: most of the energy is lost in the earliest, hardest part of the braking.

Judging the gap in front of you

The space in front of you is the only space you fully control, and it is the one that gives you time to notice, decide and brake. Judging it in car lengths does not work, because a car length is hard to picture at speed and the room you need changes with how fast you are going. Judging it in time does work, because the time it takes you to reach a fixed point is the time you have to react.

Here is the method. Pick a fixed object beside the road ahead: a sign post, a light standard, a painted line, the shadow of an overpass. When the back of the vehicle in front passes that object, start counting steadily. If you reach the same object before your count finishes, you are too close, so ease off and rebuild the gap. The guide for your province tells you the count to use in good conditions, and the number varies, so learn the one you will be tested on.

Then grow the count. Rain, snow, ice, loose gravel, fog, darkness, a heavy load, a trailer, worn tires, a long downhill and following a large vehicle whose spray or bulk blocks your view are all reasons to leave more time, and several of them together are reasons to leave much more. You will never regret the extra space. It also smooths out your driving, because a driver with room ahead brakes gently and rarely.

Being followed too closely, and moving with traffic

When somebody sits on your bumper, the instinct is to speed up or to brake at them. Both make it worse. The safer answer is to increase the gap in front of you, so that if you do have to slow, you can do it gradually and give the driver behind time to respond. Where it is safe and legal, let them past. Nothing about their impatience is your problem to solve at speed.

Driving well below the flow of traffic is its own hazard, because it forces other drivers to pass. Some roads carry a posted minimum speed for this reason, and every province has some rule about not obstructing traffic by driving unreasonably slowly. If your speed is well under the traffic around you, keep right, and use pull off areas on rural highways to let a line of vehicles through.

Speed also has to fall on ramps, in tight urban streets with parked cars, near schools and playgrounds when children are around, and through work zones where people are on foot beside live lanes. In those places, the safe speed is often much lower than anything posted, and the driver who slows early looks calm rather than slow.

Building the habit

Speed control on a road test is not judged by whether you touched the limit. It is judged by whether your speed suited the road, whether you got there smoothly, and whether you left yourself room. Examiners notice a driver who arrives at every situation already at the right speed, and they notice the one who arrives fast and brakes.

The way to practise is to check the gap deliberately every few minutes rather than only when something feels wrong. Count against a fixed object, adjust, and carry on. After a few weeks the count becomes a feeling, and the feeling is what keeps working when the test is over.