Speed Limits & Following Distance — British Columbia

Eight times as much braking power is required when a vehicle is twice the weight of a car and travelling at twice its speed. The handbook's speed and weight facts say that doubling vehicle speed alone calls for four times the stopping power, and doubling vehicle weight alone calls for twice the stopping power. When both happen together, the requirement is eight times the stopping power. That matches the situation in the question: twice the car's weight and twice its speed.

Under ideal conditions, passenger cars and light trucks should keep at least a two-second following distance behind the vehicle ahead. The handbook's basic driving skills section gives a two-second rule for drivers of passenger cars and light trucks. It tells you to watch the vehicle ahead pass a checkpoint, such as a sign post, and count “one thousand and one, one thousand and two.” If you reach the checkpoint before you finish counting, you are too close, so you drop back and try again. The same section separates heavy vehicles, which need more space, and says to lengthen the gap when road or weather conditions are poor. That makes two seconds the baseline for ideal conditions.

With good traction and steady speed, doubling your vehicle's weight doubles your braking distance. The handbook's speed and weight facts say that twice the vehicle weight calls for twice the stopping power, while weight left unchanged with speed doubled calls for four times the stopping power. Since the question holds traction and speed constant and only the weight changes, the extra weight goes up in a straight one-to-one relationship: two times the weight means two times the stopping power, so the distance needed to stop goes up the same amount. That is why the answer is doubled rather than multiplied by four or eight.

Under ideal conditions, a 17.6-metre truck should keep at least a 5-second following distance behind the vehicle ahead. The handbook groups trucks with buses and other heavy vehicles. For those vehicles it says to keep at least five seconds behind the vehicle ahead, because they need more time and distance to stop. Another passage repeats that drivers of buses, trucks and other large heavy vehicles should never be less than five seconds behind at highway speeds. The question asks about ideal conditions, so the stated minimum is five seconds; less ideal conditions call for more.

If you double your speed, your braking distance becomes quadrupled — four times as long. The handbook's speed and weight facts say that doubling vehicle speed requires four times the stopping power. Speed is the only thing changing in this question, since weight stays the same and traction is good, so the same doubling relationship applies: twice the speed means four times the stopping demand and four times the distance covered while you shed that speed. The handbook also notes that doubling only the weight calls for twice the stopping power, which shows the speed effect is the stronger of the two.

The safest drivers are those who adjust their driving to the conditions. The weather section of the commercial driving guide says good drivers make sure the vehicle is properly equipped for weather hazards and recognize when conditions are too risky to drive at all. It then describes how to adjust to ice, sleet or snow, starting by getting the feel of the road at low speed and testing steering and brakes for control. The common thread is matching what you do behind the wheel to what the road is giving you, rather than assuming a fixed style of driving works everywhere.