Emergency braking in detail

Perversely, this essential skill is not taught to learners...

In the following scenario, an untrained, inexperienced driver and an experienced, trained driver head over a crest on the highway, in the wet, in identical vehicles. Over the crest there is an obstacle blocking the road. It can be anything you really would not like to hit – a crash between two vehicles, a broken-down truck, a drunk pedestrian or a cow in the middle of your lane. When the problem becomes visible to the drivers, it is 70 metres away.


Let’s see how the two very different drivers deal with the same physical situation.

Driver One is untrained and drives with the cruise control set to the prevailing highway speed limit, 100km/h. He sees the truck, takes one second to react and slams the brakes. Result? Not good. He crashes into the obstacle at 50km/h.

Driver Two is experienced and trained. He identifies the crest as a potential hazard. So he ‘switches on’ mentally. Identifying what’s over the crest becomes a strong priority. (This is the benefit of experience.) One cannot see beyond a crest, therefore adaptation is required. So he makes a conscious decision based on intelligent assessment. No advanced skill or particular coordination is required.

Driver two applies these kinds of assessments dozens of times daily, so this is not a panic response; it is an automatic one. He probably does it without thinking consciously. It is not a complex or highly evolved reaction. He merely lifts off the throttle. Gravity and rolling resistance kicks in. The car’s speed drops to 90km/h.

While all this is going on, driver two does something else. This is where driver two benefits from his post-license driver training (what some people call advanced driver training): all he does is move his right foot across and makes light contact with the brake pedal. He does not brake; he just gets his foot over there in case he needs to brake later, over the crest, and it really cuts his reaction time. It’s another non-panic-type response, more of an ‘insurance’ move. And not one requiring much in the way of advanced motor skills either.

This simple adaptation shaves a quarter of a second from his reaction time. But it pays off. A quarter of a second at 90 kays shaves 6.25metres – more than a car length – from the stop. His ‘advanced’ training also means the seat is adjusted properly and his hands are on the wheel in the right places, at nine and three o’clock, which will be vital to swerving and maintaining control if an emergency response is required. Luckily, he doesn’t have to – as a result of getting it all right he misses the truck by six metres. That’s a lot of daylight, compared with driver one’s reaction – if that’s the right word – to the same situation.

(Physics laureates take note of the assumptions used in support of the example: This example uses a one-second reaction time for Driver One, a 0.75-second reaction time for Driver Two, the saving a result of Driver Two’s foot already being in contact with the brake. Brakes comprise a 0.7G emergency stop, notionally in the wet, under full ABS brake activation. We also assume both cars have ABS brakes.)


Human beings are not naturally proficient at stopping cars. Nor does the licensing process involve training on emergency stopping. Sadly, the first time most drivers experience an emergency stop is during an actual emergency. (You’d be rightly miffed if that was Qantas’s pilot-training policy…)

Back when rear disc brakes were considered cutting-edge brake technology, the human element was easily braking’s weakest link. An untrained panic stop meant the driver lost control of the braking process and Isaac Newton took over. Not good, since he’s been dead almost three centuries.

Much work has gone into sidelining human frailty from the braking process. Today it’s doubtful any of racing’s top guns could out-brake the technology built into a modern, well-specified car.

Braking’s three key crash-avoidance advances (aside from the continuous improvements in tyres) are:

ABS (anti-lock brakes): Some think it cuts stopping distance. It doesn’t – It allows the driver to retain steering control during an emergency stop. Since you can’t steer with wheels that have locked, ABS senses lock, then releases the brake pressure briefly, allowing the wheel to turn a little before ramping the pressure back up. The theory is, you can therefore brake hard and steer around that drunk pedestrian in the middle of the road without practicing emergency stops every second day. Sadly, a worrying proportion of drivers fear a malfunction has occurred the first time they feel ABS’s trademark lock-release-lock-release ‘chatter’ underfoot.

EBD (Electronic Brakeforce Distribution): In the bad old days, cars had pressure-limiting ‘proportioning’ valves running to the rear wheels. This limited the amount of braking the rears could do (bad) but ensured it was the fronts that locked first (good; prevented much brake-inspired pirouetting). EBD adds extra deceleration to the braking process by ensuring the rear brakes work as hard as they possibly can during every emergency stop.

EBA (Emergency Brake Assist): Some people just don’t punt the brake pedal hard enough, soon enough, when the chips are down. EBA senses a panic-style pedal press, and quickly ramps the brake pressure to achieve maximum deceleration early. The resulting fast, flat-chat brake application potentially slashes car lengths from the stop.

safetyJohn CadoganComment