Speed and Braking Distances .. are we being conned?

What you are about to see will change your mind about road-safety advertisements

Introduction

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In an effort to convince people to slow down, many Australian government authorities have attempted to show that higher speeds result in longer braking distances, and hence slower is safer. These efforts have taken the form of brochures and websites with graphs showing different braking distances, and more recently have used video of simulated braking demonstrations. The implication is that if you stick to the speed limit you will be safe, but dare to go even 5km/h faster and you will surely crash.

All this has been done in an environment of increased police enforcement of speed limits. The introduction of 50 km/h local speed limits in many states of Australia seems to have provided the impetus for even greater exaggeration.

Another trend (from the video type adverts) is that of constantly showing people video of cars screeching to a halt with all wheels locked, and therefore many will assume this is the best method of reacting to an emergency situation. They are imprinting the wrong behaviour - panic brake (and lock 'em up) instead of swerving, and braking in a controlled manner for maximum deceleration and steering control.

In NSW, a Falcon takes 37.72 metres to stop from 60 km/h (already an exaggeration) yet in Queensland the same car takes 71 metres (source)

Unfortunately, in an effort to convince the public of the dangers of speeding they have exaggerated things to the point of absurdity. Despite improving automotive technology, official estimates of braking distances are growing over time from fanciful to absolutely unbelievable!!! The most notable example of this has occurred in Queensland (see below) where braking figures for a large car stopping from 60 km/h have gone from 34 metres in1999 to 71 metres in 2000.

The most striking feature of the table below is the lack of consistency, and just how far beyond reality they have been tempted to go in order to increase the dramatic effect. You would think that if they were going to fib, they would at least stick to the same consistent story.

Location	Date approx	Identity	60km/h stopping distance (m)	Reaction time (s)	Braking Coefficient (g)	Links
NSW	6/2003	Prof Ian Johnson - Slow Down - TV	37.72	1.20	0.80	source (7Mb) comment
QLD	-1999	Speed Facts - Official website and brochures	34.00	0.75	0.60	source comment
QLD	1999-	50k's - Boy with model aeroplane - TV	50.63 63.73	1.50	0.553 (12m) 0.366 (16m)	source comment
QLD	2000-	Speed Facts (small car) - Official website and brochures	48.00	1.50	0.615	source comment
QLD	2000-	Speed Facts (medium car) - Official website and brochures	56.00	1.50	0.448	source comment
QLD	2000-	Speed Facts (large car) - Official website and brochures	71.00	1.50	0.31	source comment
QLD	2002-	Every (k) over is a killer (3 red commodores) - TV	66.68	1.50	0.34	source comment
SA	1997+	Road Accident Research Unit	45.25	1.50	0.70	source comment
SA	2003	50k's introduction - radio and TV	45.25	1.50	0.70	source comment
VIC	2003	TAC website	45.00	1.50	0.71???
NMAA	2003	Real world tests	24.08-28.34	0.5-0.7	0.85-0.90

Explanation of the technical aspects

We began our investigation after seeing the Slow Down advert that arrived on NSW screens in June 2003. The professor was good enough to supply a copy of the spreadsheet used to perform his calculations and we were able to confirm that his formulas were all ok, but his assumptions about reaction time and braking performance were rather conservative.

Using these same equations we were able to assess a whole range of "road safety" campaigns and we were astounded by the wild variations between different campaigns. (see below)

Equation of motion (layman's version)

In simple terms we start off with a car travelling down the road at a certain speed (Vi), the driver sees something happen and takes a moment to react and take action to avoid a crash (reaction time, Rt) the brakes are used to slow the car down at a constant rate of deceleration, represented relative to the force of gravity (coefficient of braking).

If we plug these three variables into the equation we can compute how long the car will take to stop, and what speed it will be doing along the way (and hence the impact speed if there is one)

Coefficient of braking

In high school physics you may have learned about 'coefficient of friction' when dealing with a mass skidding along a surface. We are using the same model here, and for the sake of simplicity we are lumping all of the characteristics of the car (tyres, brakes etc) together and calling it the coefficient of braking. The higher the number the better.

A little research, looking at road tests in car magazines allowed us to confirm the following typical values.

Reaction times

Having seen the official figures go from three quarters of a second to one and a half seconds over time again made us just a bit suspicious. Looking more closely at the mathematics, it becomes obvious that as you go slower, a greater percentage of your time is spent reacting than actually braking. If you want to accentuate the difference in stopping distance between two speeds (e.g. 50 and 60 km/h) the longer the reaction time, the more dramatic the difference will be.

A quick scan of the research reveals that a lot of people do, in fact, react in 0.75 seconds or less, but we figured it would be better if we could prove it. Using an excel spreadsheet with a few simple macros, it is possible to do just this (try it for yourself)

NSW advert (July 2003)

Queensland Transport - until 1999

Internet archive of http://www.roadsafety.net/Speed/html/facts.html

THE EFFECTS OF SPEED It might not seem like very much, but an increase of between 10 -15 km/h can mean the difference between life and death in some circumstances. REACTION / BRAKING DISTANCE After realising that there is an emergency, the average driver takes about three-quarters of a second to react. Your vehicle can travel a long way in that time. Copyright Queensland Transport 1998

Queensland Transport had been quoting the above (somewhat exaggerated) figures for years, right up to about the time that they wanted to justify bringing in the 50km/h default urban speed limit. Suddenly we were bombarded with images of children almost being killed by the evil speeding driver and an even more fantastic assessment of the typical car/driver's braking capabilities.

It is laughable how much the "official figures" suddenly changed.

Queensland Transport - 1999 onwards

http://www.roadsafety.net/SPEED/facts.html

The facts It might not seem like very much, but an increase of between 10 -15 km/h can mean the difference between life and death in some circumstances. After realising there is an emergency, the average driver takes about 1.5 seconds to react. Distances shown are for a car equipped with the best possible tyres and brakes and driven by a professional driver. © Copyright Queensland Transport 2000

We at NMAA would really like to meet the "professional driver" used to fudge the above figures.

In the road tests we have read there is no discernable trend for large cars to perform worse, as manufacturers usually fit larger tyres and brakes accordingly. In fact it is usually the smaller "economy" cars that have the poor braking performance due to their economy brakes and matching tyres. A roller skater doing 60km/h could stop in a shorter distance than the 71 metres stated above.

The above figures are nothing short of ludicrous!

 

This advert appeared on Queensland television in 1998. It was part of the softening up process to prepare the public for the introduction of 50 km/h default urban speed limit in south-east Queensland. We see a boy with a model airplane run onto the road oblivious to the danger. Then we see a Falcon screech to a halt just stopping in time. Next a shadow car (supposedly doing 60 km/h this time) screeches to a halt almost 3 car lengths further down the road. The voiceover states: "A car travelling at 50 kilometres an hour will stop twelve to sixteen metres before one travelling at 60, which can make all the difference. From March 1 you must not exceed 50 kilometres and hour on local streets, unless they're signed otherwise. So now the law says 'slow down to 50'; 50 on local streets will save lives." Using their recommended 1.5 second reaction time and solving the speed/distance equations for a 12 metre difference between 50 and 60km/h stops we get a braking coefficient of 0.56 and a 60 to 0 stopping distance of 50.31m (bear in mind that the real difference is only about 6-7 metres - this is still significant but why do they feel the need to exaggerate everything?) We also note that the correct response in this situation would have been to swerve around the child, since only a 2 metre deviation would be required to guarantee that the child would survive no matter how fast the car was travelling. The full video can be downloaded from the Queensland Transport website. http://www.roadsafety.net/showcase/mpegs/50k.mpg

Queensland Transport - TV advert 2003

South Australia

When SA decided to bring in the 50 km/h default urban speed limit, they ran a series of adverts on TV and radio quoting the following:

Speed	Reaction Distance	Braking Distance	Total Stopping Distance	Reaction Time	Braking Coefficient
60 km/h	25 m	24 m	49 m	1.5 seconds	0.7
50 km/h	21 m	18 m	39 m	1.5 seconds	0.7

These are the same reaction time and coefficients that appear in the ubiquitous reports that come from SA's own Road Accident Research Unit (RARU) that has been responsible for much of the so-called research upon which many speed limit policies have been based in recent years. Further reading brakeswerve, 50k myths

Victoria

The Victorian Transport Accident Commission advises on their web site thus:

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