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Unread 02-08-2017, 10:10 AM   #1
Join Date: Aug 2015
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Default Emergency Braking

When last I wrote here, I addressed the subject of limit-handling. A much more common sub-type of such maneuvering is sudden braking. Even the most vigilant of drivers can be made to stop in a hurry, yet very few drivers actually manage to perform this simple task.

Most cars today have ABS (and auxiliary systems like EBD) and can be braked as hard possible, as quickly as possible while still staying around the threshold of lock-up and allowing to steer or continue to negotiate a turn.

This is important because a situation can transpire where the braking capability of the car will be insufficient, and the driver will have to steer around an object. Since the success of a steering maneuver is dependent on speed, braking first, to slow down as much as possible, is still important. ABS allows you to brake and then steering while still braking.

Nevertheless, few drivers actually brake as hard as possible in the moment of truth. They are so hesitant as to the magnitude of the car's reaction, that they brake progressively.

Practice stomping on the brake pedal in an empty lot a couple of times. In the worst case, braking too hard can be alleviated by easing off of the brakes. Braking too lightly, however, can't always be rectified.

Sans ABS
Without ABS, this kind of braking will of course lock wheels. But is that really a problem though? Locked wheels still stop the car pretty damn quickly, at least on anything that more grippy than smooth ice or compact snow.

As for steering: Well, if you don't need to steer (which, if you brake, you will often not need) than you can keep the wheels locked. If you do - brake to lock-up, ease off to steer and than reapply until you stop.

"But what about Threshold Braking?" you probably ask. Surely that is superior to just locking the wheels, right? Well, technically yes. But Threshold-Braking is a motorsport technique, which means that it is a specialist technique (and even the very best don't get it right every time), and that it is used in a very controlled environment, where you often know exactly when and where to brake.

On the road, in a situation with an element of surprise - you probably won't manage it, and your fumbling attempts will only elongate the stopping distance, where locked wheels will produce a stopping distance not much longer than Threshold Braking.

The only trick that can shorten the stopping distance on the public road is to stomp on the pedal first, and than start to ease off progressively until you find the threshold and hang on to it. But even that sort of braking required a lot of coaching and practice.

The everyday meaning of braking distances
When I coach, practicing an emergency stop and stopping and steering is one of the first things we do. That is because the stopping capabilities of the car and driver effect two important elements of road driving: The first being choice of speed.

A safe speed is determined predominantly by asking oneself "at this speed, do I estimate that I be able to safely stop well within the area seen to be clear?" If you can see 80 feet ahead, than you need to drive at a speed where you brake relatively firmly to a stop and still have a few feet to spare.

The same applies in the lateral sense: Imagine a car parked at the side of the road which can hide a pedestrian about to leap unto the road. Can you stop besides that car at the point where you are just starting to see around it?

Now, in assessing the stopping distance, one must keep reaction time in mind. Generally, an alert driver will react after a whole second, and a distracted or fatigued driver will react after two or more. This is important for the other element I spoke of, which is following distances.

Following distances are supposed to prevent rear-ending another vehicle even if it is suddenly braked to a stop or if something falls off of it. So, you keep enough margin to account for your reaction time, and a reserve: typically a margin of two seconds overall, but it can be more depending on your vehicle's stopping capabilities, your reaction time and the car behind you.

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