Experienced truck drivers know that a truck’s stopping distance is much more complex than just response time and truck speed. There are five primary environmental factors that can impact stopping distance, and knowing how to respond to them is key to controlling your vehicle.
Traction is a measure of a tire’s ability to exert a force on the road surface, changing the truck’s motion – direction and/or speed. Slippery surfaces reduce traction and a tire’s ability to exert the force needed to control the truck – which increases braking distance. Wet roads can double the time it takes to stop your truck, and of course the slicker the surface the longer it takes to come to a stop at a given speed. Truck drivers can’t control the weather, you can control your speed; and a slower speed can offset a loss in traction. These recommended speed reductions are based on the slickness of a surface:
- Rain, water, fog: reduce speed by 1/3 (e.g., slow from 55 to 35 mph)
- Packed snow: reduce speed by 1/2 or more
- Ice: Don’t drive — slow to a crawl and stop driving as soon as you can safely do so
Here’s a cold weather tip: open the window and feel the front of the mirror or antenna. If there’s ice on either, the road is probably starting to ice up as well. You can also watch passing vehicles – if the road looks wet but there’s no spray, the road is most likely turning icy and black ice is forming. Remember – bridges and overpasses ice up first.
When a truck is turning, a portion of its tires’ traction goes into changing the truck’s direction. Because changing a truck’s speed also requires traction, there is greater risk of losing traction in a curve while trying to change the truck’s speed – and when a truck loses traction in a turn it continues straight, right off the road. So truckers should always slow before a turn and gently accelerate out of it whenever possible. When reducing speed before a turn, truckers should note that often their speed should be less than the posted maximum safe speed given the higher center of gravity, because in some cases the tires may keep traction but the centrifugal force could tip the load and roll the truck.
Another key to maintaining a safe stopping distance is visibility – if you can’t see, you don’t know braking is necessary. Truck drivers should always be prepared to stop within the distance visible from the cab, and if visibility is reduced because of rain, fog, following distance or darkness speed should be decreased to reduce total stopping distance.
In heavy traffic, the safest speed for a truck is the legal speed of the vehicles around you. It’s simple – vehicles are less likely to run into each other when travelling at the same speed and in the same direction. But truckers should be aware that many states reduce the speed limit for commercial trucks, sometimes as much as 15 mph, so a simple match isn’t always possible. When this is the case, use extra caution while changing lanes because this difference in speed is a potential hazard for vehicles approaching from behind.
The force that gravity exerts on a truck comes into play on hills. On an uphill, a truck’s stopping distance is reduced because gravity pulls the truck and its load backwards. The opposite is true downhill – where gravity pulls the truck and its load forward, increasing a truck’s stopping distance. Knowing this, the best approach for a driver is to select and maintain a speed that is not too fast for:
- The total weight of the truck and its load
- The length and steepness of the downhill grade
- The weather and road conditions
And experienced truck drivers know to use the braking effect of the truck’s engine as an additional method of controlling downhill speed. Slowing to an appropriate safe speed and shifting to a lower gear before starting downhill will save brakes for when you have to come to a stop and will help prevent overheating and brake fade – especially over long, steep downgrades.
Driving too fast is the number one cause of fatal crashes. Experienced operators automatically adjust the speed of their truck in response to changing road conditions. Controlling the speed of your vehicle is your first line of defense against uncontrolled braking emergencies. Stay alert, know the weather, and be aware of changing driving surfaces.