Stopping distance

As benjya notes above, the UK driving test requires applicants to memorise the data published in the Highway Code. As he further hints, the actual stopping distance of a real car may well be substantially different from the nominal figures published in the Highway Code. He's right to suppose that the Highway Code data is both incorrect and incomplete.

Most experienced motorists also know that stopping distance depends on the type of car; the road surface, the make and condition of the tyres; how heavily laden the car is, and whether it is going uphill or downhill and a stack of other factors. This article aims to fill in some of the gaps left by the Highway Code.

Deceleration under maximum braking is an important aspect of both safety and performance in any vehicle. For example, in auto racing one of the key techniques of winning drivers is to out-brake your opponent. That is to say, leave braking to the last possible moment before a bend, and then brake at full force. This allows a driver to cover the same distance a little quicker than a less courageous rival. And, if you ask a racing driver which is more important: fast acceleration, or powerful brakes, a lot of drivers will go for good brakes before good acceleration.

Motorists also know from daily driving experience that grip is less on ice, or on a wet, greasy surface. Less grip means less deceleration and longer stopping distances.

Deceleration is key to stopping distance

Modern automotive data suggests that 38 metres is the shortest distance a VW Golf-type car can possibly cover when stopping from 100 kph (excluding all thinking time) That means that from the moment the brakes are applied, from a constant 100 kph on a flat road, using a standard road surface, the car will come to a halt in an absolute minimum 38 metres. This assumes that a car will decelerate at a rate of 1 g (or 9.81 m/sec) under emergency conditions.

To achieve this minimum distance, tyres and brakes must be in good condition before the test, and the driver must have the strength to apply the brake pedal very firmly. No electronic assist systems are needed.

The Highway Code claims that the total stopping distance from 60 mph (a little slower than 100 kph) is 240 feet, made of up 60 feet thinking distance and 180 feet stopping distance.

Because the Highway Code is in GodAwful Imperial units, I'm going to work in feet per second: 60 mph is 88 feet/sec (60 * 5280/3600). The Highway Code reckons that thinking distance is 60 feet, which works out at 0.68 seconds. That's pretty optimistic, according to most international hIghway safety authorities.

The HIghway Code further claims that the car can slow from 88 feet/sec to zero in 180 feet. v2=2as, so the acceleration is v2/2 = 882/360 = 21.1 ft/sec2. Now g, the acceleration due to gravity is 32 ft/sec2, so the Highway Code expects a deceleration of about 66 percent of g.

This is a low figure, and seems to support gbulmer's suggestion that the Highway Code numbers are based on an old Ford Anglia with drum brakes. Either that, or they are supposed to represent realistic conditions (hesitant drivers, poor brakes and wet roads) rather than perfect conditions.

In good conditions, therefore: with an alert driver, good tyres and on a good road surface, the stopping distance will be a great deal less than the HIghway Code suggests. Even allowing one second for a driver to assess the situation, realise the brakes need to be applied and press the pedal to the metal, the overall stopping distance under ideal conditions is likely to be around 68m, or 210 feet, compared with the HIghway Code's estimate of 240 feet.

Better brakes mean shorter stopping distances

Just as racing cars have better brakes than your average saloon, cars designed for sporty driving have better brakes than low-performance vehicles. You might expect a Ferrari or Porsche to be capable of stopping before a Toyota Corolla travelling at the same speed. You would be right.

Here is some data from an Australian driving school, using amateur drivers:

Car					90kph	(a)	120kph	(a)

Ferrari 550 Maranello			33.6	94.8%	59.7	94.9%
Mercedes C36				36	88.5%	63	89.9%
Mercedes SLK230 Kompressor		36	88.5%	62.7	90.3%
Saab 9000 Aero				36.6	87.0%	66.3	85.4%
BMW Z3 (2.8)				36.9	86.3%	64.5	87.8%
Porsche 911 Carrera 4			37.8	84.3%	66.9	84.7%
Nissan 200SX				38.7	82.3%	68.4	82.8%
Subaru Liberty RX			40.8	78.1%	70.8	80.0%
Honda Integra GS-R			42	75.8%	74.4	76.1%
Lexus ES300				42	75.8%	73.8	76.7%
Nissan Maxima				42	75.8%	72.9	77.7%
Audi A4					43.5	73.2%	80.7	70.2%
Toyota Camry V6				43.5	73.2%	82.2	68.9%
Lexus LS400				45.3	70.3%	78	72.6%
Mazda MX-5				45.6	69.9%	76.8	73.7%
Mazda Protege				47.4	67.2%	86.1	65.8%
Toyota Corolla				55.8	57.1%	95.7	59.2%

The table shows the stopping distance in metres from each of two speeds, with the deceleration (a) in percent of g. Note that the Ferrari is braking at just under 1g, but the Corolla only managed less than 60 percent of 1g.

Stopping distance to reduce still further

Back in 2000 or so, Continental AG, a manufacturer of tyres and automotive electronics, shocked the car-making industry by producing a VW Golf capable of stopping from 100 kph in under 30m, using the same, standard conditions. This is equivalent to a deceleration of 131 percent of g, and is comparable with performance of many racing cars. Compare that figure of 1.31g with the industry-best figure of 1g (39m) and a typical real-world situation of 0.7g ( 56m).

Ultimately, the deceleration under emergency braking depends on two key factors. The first is the grip between the tyres and the road surface and the second is the amount of torque that the brakes can apply to a rotating wheel. A third factor is the force that the driver can apply to the brake pedal, and hence transmit to the brake mechanism.

Smart readers will claim that the mass of the vehicle and occupants is another significant factor. However, this is not such a no-brainer as it might at first appear. First, the frictional force available at the tyre-road interface increases as the applied weight. Both the stopping force and the deceleration rate depend on vehicle mass in a more or less linear way. In laymans' terms, this means the two effects cancel each other out and weight should not have much of an effect on the stopping distance. In reality, the driver frequently does not apply maximum braking force, or the brakes are not good enough to apply sufficient stopping force, so a heavy vehicle often does take longer to stop.

Regardless of vehicle weight, the traction between tyre and road and the available braking torque at the wheel have improved hugely over the years, so that it is has become difficult for many drivers to apply the maximum stopping power of a modern vehicle. In addition, research shows that many drivers are hesitant, even in clear-cut emergencies, and do not push the brake pedal sufficiently hard.

In hydraulic braking systems (normal on most vehicles, except heavy trucks), the harder you push on the brake pedal, the more braking force you apply to the wheels. It is physically difficult for full-grown adult males to apply sufficient pressure to deliver maximum braking force, let alone weaker individuals. For this reason, car makers have devised Emergency Brake Assist (EBA) systems. These are tied into the vehicle drive and transmission electronics, along with ABS, traction control and electronic stability control (anti-skid) systems. When the electronics detect that the driver is attempting an emergency stop, by the rate at which the brake pedal is depressed, the EBA kicks in, and brings a small pressure reserve to apply maximum braking effort.

Typically EBA will reduce stopping distance by 2m (from 11 to 9m) at 30 mph; by 5m (30m to 25m) at 50 mph and by 10m (60m to 50m) at 70 mph. Just to keep things consistent, here are the deceleration numbers taken from a Ford Mondeo, with and without EBA, using a trained driver.

				No EBA			EBA
MPH	(kph)		m	percent g	metres		percent g	
30	(52.0)		11	96.6%		9		118.0%
50	(86.6)		30.5	96.7%		25		118.0%
70	(121.3)		59.5	97.2%		50		115.7%

Naturally, by applying maximum braking force, there is a risk that the wheels will lock, so EBA comes only on cars fitted with ABS systems. The two technologies work together to apply maximum braking force where the tyres can take it, and release the brakes when the wheels start to lock. Together, they can significantly improve the stopping distance of your car, and allow drivers to retain full control through the steering wheel. As the man said, "A significant improvement to road safety"

There is no doubt at all that braking potential will improve significantly in the near future. Instead of viewing tyres as commodity items that can be fitted at a late stage in vehicle development, car makers are sponsoring a surge of research into how tyres and brakes and suspensions can work together to deliver maximum performance and safety. As brake manufacturers realise what tyre makers can do, and vice versa, stopping distance will reduce still further in future vehicle models.

One example of this is the predictive brake assist developed by Bosch. In this, the electronics try to predict when the driver may try emergency braking, and the system builds up a reserve of pressure in the brake fluid, ready to push the brakes hard onto the wheel, the moment the brakes are applied.

According to Bosch, Even in critical traffic situations, only some 30 per cent of drivers will initiate a full braking action, most drivers are too hesitant.

Warning! Many websites and drivers comment that the vibrating sound and feel built into ABS systems causes drivers to panic and possibly even lift off the brakes, in the mistaken belief that the brakes are faulty. The vibration is there to warn the driver that grip is low, but it appears to have the effect of making them think there is something wrong. This shows (I think) that ABS is very rarely used, indicating that tyre-road grip is usually far greater than most people imagine. Thanks to themanwho

Thinking time

No matter how hard tyre makers and brake systems experts refine their hardware, stopping distance depends critically on how quickly a driver reacts to an emergency situation.

In the interval between an incident occuring, and the driver hitting the brakes, the car is travelling at full speed toward the incident. If the driver fails to hit the brakes, then no amount of electronics will stop the car.

This interval is called the thinking time, and can be divided into two main components.

  • Perception time
  • Reaction time

The reaction time is the most consistent of these two elements. In countless tests of alert individuals, the time it takes from perceiving an event to your muscles reacting to it, is between 0.15 and 0.3 seconds. In a car, there is an instant longer to switch feet from the gas pedal to the brake (in a manual transmission) or to adjust the position of the feet (in an auto) but it takes a maximum of about 0.4 seconds from the a stimulus triggering the the brain to action, until brakes are applied.

Of course drugs, drowsiness and alcohol are well-known to reduce reaction times, and an individual affected by any of these might add another 0.1 or 0.2 of a second before hitting the brakes.

Perception time is a whole different matter

Clearly, if your eyes are not watching the road, there is no possibility of perceiving a dangerous situation ahead. Even if you are watching the wrong part of the road, you may miss a potential problem. Motorcyclists, for example, are taught to constantly shift their area of gaze and concentration from near the front wheel to the horizon and to the right and left, looking for potential hazards.

However, data from road safety organisations appear to show that perception times of half a second are fairly typical. Add that to a third of a second reaction time, and the total time from seeing an event to hitting the brakes is around 0.7 seconds. With alcohol that increases, quickly approaching a full second with each extra shot.

In one of the few test results I could find on the web, an Irish motoring organisation tested four young drivers both before and after consuming alcohol. Even before the alcohol reaction times were close to a second.

We took four students - Lucinda Andrews (19), Aisling Flinn (20), Shauna Frey (19) and Peter Wallace (18) - to the RAC School of Motoring and got them to do the emergency-stop exercise "stone cold sober". Then we took our volunteers, all learner-drivers, to the pub across the road where they each consumed three to four drinks (including alcopops and cider). We then went back to the simulator and repeated the emergency-stop test …

The results were shockingly conclusive. Our volunteers performed the emergency-stop test at three different speeds: 20mph, 40mph and 60mph. In all but one of the post-alcohol tests, reaction time was markedly slower and stopping distance was considerably longer.

At 60mph, for instance, Lucinda's reaction time slipped from 0.822 to 0.998 of a second, increasing her stopping time from 53 to 70 metres - almost 20 metres of a difference. "I was really surprised by how much my driving deteriorated," she admitted afterwards. "The test has been a lesson to us all, and it convinced me that there's no 'safe' amount of alcohol you can drink and still drive safely."

Name		test	reaction (sec)	distance

Peter		Sober	0.622		32.3
		Drunk	0.857		41.1

Aisling		Sober	0.658		34.1
		Drunk	0.908		45.2

Lucinda		Sober	0.593		28.1
		Drunk	0.897		42.3
Shauna		Sober	0.562		27.2
		Drunk	0.757		36.0

In another for the US NHTSA, drivers were tested in a simulator and on a test track. The purpose of the experiment was to prove that the simulator was a good method of testing drivers. It proved that reaction times were very similar, both on the track and in the simulator, but it also showed that drivers were taking about one second to release the accelerator pedal; 1.6 seconds to begin steering and over 2 seconds to hit the brakes. The incident was a large foam mock-up of a car, propelled onto the road in front of the driver.

So it seems reasonable to assume that in real-life situations many drivers will take up to 2 seconds to fully react to a potentially dangerous situation. At 100 kph this corresponds to around 55m. Compare that with the 38 metres or so taken to stop the car from the same speed, and it is clear that improving driver reaction times needs to be a priority if road safety is to be improved.

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