Deceleration (coasting)

Every trip has a destination with stops along the way, whether the trip is a short visit to the grocery store or a trip across the country. These stops require deceleration, or slowing down, of the vehicle, and sometimes coasting. Knowing how to decelerate and when to coast is essential for operating a vehicle safely.

The Merriam-Webster Dictionary defines “decelerate” as “to reduce the speed of” or “slow down.” The dictionary defines “coast” as moving “as by momentum or gravity.” Thus, deceleration can include braking, while coasting involves lifting the feet off the pedals and allowing the vehicle’s momentum to carry it forward.

One way of slowing down is shift gears into neutral and to allow the vehicle to slow down slowly. Some drivers believe that this technique can save on fuel, this reducing the costs of running a vehicle, as well as helping to reduce pollution. However, as the Popular Mechanics website explains, the engine is still working, even in neutral, and the fuel savings are generally quite minimal. Shifting to neutral can also potentially be dangerous if the driver must suddenly react to a hazard on the road. The extra second or two that it takes to shift back into gear could make the difference between crashing or being able to maneuver out of danger.

How to Stop

In manual vehicles, drivers have three basic options for coming to a stop. They can shift into neutral and coast to a stop, they can shift down one gear at a time, or they can wait until they are close to the place where they need to stop and apply the clutch and the brakes without this intermediate step. As Jason Tchir explains in his May 2017 Globe and Mail article, shifting into neutral too early can cause additional wear and tear on the brakes, making them need repairs earlier or possibly making them ineffective. However, even the experts disagree on the best method to use.

In manual vehicles, drivers have three basic options for coming to a stop. They can shift into neutral and coast to a stop, they can shift down one gear at a time, or they can wait until they are close to the place where they need to stop and apply the clutch and the brakes without this intermediate step. As Jason Tchir explains in his May 2017 Globe and Mail article, shifting into neutral too early can cause additional wear and tear on the brakes, making them need repairs earlier or possibly making them ineffective. However, even the experts disagree on the best method to use.

Automatic transmission vehicles have somewhat more limited options for coasting to a stop. Simply lifting the foot off of the accelerator is the simplest and most straightforward option, but drivers can also shift into the D1 or D2 gears, or possibly into Neutral. However, this option has the same potential issue as with manual transmission vehicles: the extra time needed for shifting back into gear in case of an emergency. The safer option is to keep the vehicle in the correct gear as it slows down.

Coasting to a stop when possible can be a good choice if the conditions of the road allow for it. Although it may not save much fuel, it helps to reduce wear and tear on the brakes while giving other road users a better chance to anticipate an upcoming stop. On icy or wet roads, coasting to a stop can be a safer option than applying the brakes and possibly going into a skid.

Coasting and Stopping Distance

Coasting to a stop when possible can be a good choice if the conditions of the road allow for it. Although it may not save much fuel, it helps to reduce wear and tear on the brakes while giving other road users a better chance to anticipate an upcoming stop. On icy or wet roads, coasting to a stop can be a safer option than applying the brakes and possibly going into a skid.

One of the disadvantages of coasting is that it requires a considerable amount of space, far more than regular braking. According to the Government of Alberta’s web page on stopping time and distance, every stop includes three basic elements: perception time, reaction time, and braking time. At 30 kilometres per hour, these three elements are approximately equal, while at 100 kilometres per hour, braking time is approximately twice as long as the other two combined.

The time and distance needed for coasting to a stop is harder to predict, largely because of the effects of gravity and momentum. Coasting down a hill will be considerably faster than coasting on a flat road, while coasting to a stop in a large vehicle will take much longer than it does for a smaller vehicle like a car. As the Ontario Ministry of Transportation website explains, a large vehicle like a tractor-trailer can take twice as long as an average car to brake to a stop due to the weight propelling it forward. Coasting can take even longer, especially with a heavy load in the trailer.

When a vehicle is travelling at high speeds, momentum will carry it forward for a much longer distance than a vehicle going at low speeds. Similarly, a vehicle travelling downhill will take longer to stop than one travelling on a flat road or going uphill, where the forces of gravity work to slow the vehicle down rather than speeding it up.

Weather and Other Considerations

Other forces can also be at work in determining how quickly a vehicle can stop. While ice, snow, and rain can increase stopping time, a strong wind can push a small vehicle forwards or slow it down, depending on its direction. Although it can be difficult to guess beforehand how much this factor will affect a vehicle’s stopping time, it is an element to consider on the road.

Although it is quite a rare occurrence, brake systems occasionally fail. As the Auto Evolution website notes, this kind of failure often happens when the brake pads wear out or the brake fluid is too low, either due to a leak or because the vehicle’s owner has neglected to have the fluid reservoir refilled. This process could be gradual or sudden, making a slow coast to a stop the only option. However, regular maintenance can help reduce the dangers of a braking system failure.

Coast Mode

Some newer vehicles have another option for moving forward on the road. As the Torque website explains, Coast Mode is a feature which disengages the transmission or gearbox from the engine, much like shifting a manual transmission vehicle into Neutral while continuing down the road.

This feature reduces fuel consumption by a small amount, and it has the advantage of being controlled by the accelerator. Thus, the driver can disengage Coast Mode simply start using the foot pedals again instead of needing to shift gears. This feature is not available in all vehicles, but for drivers who have access to it, features like this one can help.

Stopping a large and heavy vehicle is about more than just using the brakes. With various deceleration and coasting techniques, drivers can save fuel while making their ride smoother and easier.