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Safe Speed Background Page
The thinking behind our thinking...

Speed limits are a very poor substitute for
Safe Speeds.

Road safety entirely depends on "safe speed behaviour".


 
What is a safe speed? How does a safe speed work? 
A safe speed is one that's never too fast to stop in time.

The guiding principle of setting a safe speed is contained in the phrase: "Always ensure that you are able to stop within the distance you can see to be clear." Latterly several other conditions have been added for reasons which we'll discuss. We say you should be able to stop comfortably. Roadcraft has added the condition on your own side of the road. Sometimes your braking area will be clear ahead but a hazard enters it and an accident results. This suggests that we need to add another condition: within the distance that you know to be clear and will remain clear.

So we get: The safe speed rule:

Always ensure that you can stop comfortably, on your own side of the road, within the distance that you know to be clear.

The faster you go the more difficult it becomes to maintain that knowledge, because the size of the safe braking zone increases with speed. Equally, travelling more slowly reduces the size of the safe braking zone. So we should just limit traffic speeds to make the braking zone small enough and then the roads will be safe? No. It just doesn't work like that. On that basis the only safe speed would be zero mph, and that won't get us to our destination. There are lots of circumstances in an average car journey where stopping is required, and if the driver fails to stop an accident is automatic. (imagine simply coming up behind a traffic jam).

So in normal driving all our drivers are making judgements all of the time about their safe stopping zones. In the vast majority of cases they do a decent job of it too.
 

But why "comfortably"?

Adding the word comfortably really means that in normal driving you should be able to stop within the distance you can see to be clear without panic braking. This makes a great deal of sense simply on the basis of reducing stress on your vehicle and passengers, but the more important reason is that it provides a substantial margin for error. Full panic braking in the dry on the flat probably delivers about 0.9g of braking effort. Our recommended safe zone calculations (table below) are based on half of that figure. On this basis maximum normal braking of 0.45g provides for safe driving plans, with a very considerable margin for coping with a hazard that you may not have assessed correctly.
 

But why "on your own side of the road"?

If a driving plan involves allowing for the behaviour of oncoming traffic, you then have to allow for their speed as well and effectively look twice as far ahead. Consider the following example: We are travelling at 30 mph on a single track road, we know (from the table below) that we will take 111 feet to comfortably come to a stop. We will have to stop if an oncoming vehicle appears, but so will the oncoming vehicle. It follows that the oncoming vehicle will also take 111 feet to stop, therefore we need to take immediate action when an oncoming vehicle appears 222 ft away. Whenever we risk coming into conflict with an oncoming vehicle we must be able to stop within half the distance we can see to be clear.

Typical driving does not involve coming into conflict with oncoming vehicles, and the condition "on your own side of the road" affirms that our driving plan does not involve such a conflict.

Note this special case: For single track roads and some overtaking you must be able to stop comfortably within half the distance that you know to be clear.
 

But why "know"?

The word know is added to remind us that it is not enough to have a clear space in front for braking. Our safe braking zone must remain clear of obstructions during the time we are depending on it to provide safety. One example would be that a vehicle ahead is waiting to turn across our path. We initially plan to stop before we reach the point where the waiting vehicle would be directly ahead of us. If we get some clear confirmation that the vehicle is going to wait for us to pass we can continue to drive past. If the vehicle obstructs our path we can stop safely. The we must also consider the spaces ahead which we cannot see into, yet which might hide another road user who could possibly move into our safe braking zone. This is sometimes know as masking and is explained further in the column on the right.
 

The bend exception.

One regular driving activity calls for a different method for setting a safe speed. Consider a sharp bend on an open country road. In this case there are no visible hazards other than the bend itself, and there is excellent unobstructed vision of the entire bend and a long section of road beyond. In this case you must set a safe speed  within the cornering capabilities of your vehicle. Being able to stop is not the issue.

Most bends that you encounter on British roads will not have the excellent vision of the example above. Where your vision is restricted (for example by hedges) it is normal for the safe speed according to the distance you can see to be much lower than the maximum safe cornering performance of your vehicle. These typical bends must be driven with our usual speed setting rule.

(Thanks to David Pipes for reminding me that I'd not explained "the bend exception")
 

The importance of the safe speed rule

Try this imaginary experiment. Think of leaving your house, getting into your car and driving off. We're going to use the speed limit as the main arbiter of safety, and pretend that we don't need the safe speed rule. To make life a bit easier we'll allow stopping at junctions, traffic lights and for stationary traffic ahead. As we drive off we'll accelerate up to the speed limit. We won't be reducing speed for ordinary road hazards. Think your way through the route you might take, and the hazards you might encounter. How many miles will you be able to drive before you crash? Less than five, we bet.

Now ask yourself how many miles you've really driven without having an accident. The probable huge difference between the two numbers represents the relative importance of safe speeds and the speed limit. Every time you drive, when you adjust speed for hazards (including junctions, traffic lights and stationary traffic by the way) you are using the safe speed rule. Every driver uses it every day. This is why we say that safe speeds are more important than speed limits.

It's hardly surprising that greater and greater speed enforcement is spectacularly failing to deliver road safety improvements.

Can drivers really know that a safe braking space will remain unobstructed?

Yes. Without a doubt skilled drivers can.

As drivers develop their skills in hazard perception and anticipation it becomes habitual to reduce speed in the vicinity of risk. With experience and training drivers can become extremely skilled at ensuring safe spaces all around their vehicles. Drivers with such skills can use a very wide range of speeds in complete safety. At the present time such drivers are in the minority, perhaps as few as 3% to 5%. There is no reason that we shouldn't encourage the growth of this safest group. 

But what about the less skilled drivers? We divide them into three main groups.

Novices

At any given time there are about 3 million novice drivers on the road. Novices are generally those with less than about 100,000 miles of road driving experience. It takes time to become experienced and develop good hazard perception. The speed limit is a good guide for the novice drivers, and keeping to the limit will improve their chances of not making a tragic mistake. As their experience accrues, they become more and more efficient at setting a safe speed in accordance with the conditions. 

Adequate drivers

The vast majority of the UK driving population falls into this group. They generally drive with reasonable care and set their speed safely according to the conditions. They use the safe speed rule continuously and unconsciously, adjusting their speed to take account of road hazards. These people can make mistakes, and their driving could be improved with training, but in general they meet the standards of safety that are accepted as reasonable by road users. Around 75% of drivers are in this group. 

Dangerous drivers

Some drivers simply don't act responsibly, or take sufficient care. These people may assume that the road is clear when it isn't, may violate important traffic regulations and may cause an entirely disproportionate number of accidents. This is the group that needs training (or the attentions of the Police!) most. They might well ignore the safe speed rule and put others at risk. Perhaps 10% of drivers fall into this category for one reason or another. Drunks and joyriders are included.
 

So now we have fully 85% or more of the driving population, and we can generally trust them to set their speeds properly and with safety in mind. They may not be fully competent to use the highest speeds, but 100 mph on a clear motorway in a suitable vehicle is not dangerous for this group.
 

How does this all work in practice?

Our skilled driver uses experience and especially hazard perception to scan the entire road situation ahead. He is looking especially for obstructions to his path (hazards) and also items that could move into his path and obstruct it before he gets to them (potential hazards). The skill in recognizing and correctly assessing hazards ahead is called hazard perception and develops with experience at driving. It is said that most drivers take perhaps 100,000 miles of experience to become really proficient at hazard perception. There's a third class of risk that the advanced driver always takes account of and that is masking. Suppose a lorry is parked ahead on the left. We can't see through it so we assume the worst and imagine that a potential hazard is masked by the lorry and will force us to stop. The same rule applies to any area ahead that you cannot see into. If it might hide a potential hazard assume that it does (and that we will have to stop) until you can properly see into the masked area.

Once a hazard ahead has been recognized the advanced driver has choices about how the hazard is neutralized. He may choose to stop safely before he gets there. He may choose to slow and wait for the hazard to clear or he may decide on a safe course around the hazard.

Here's a typical potential hazard neutralizing sequence:

We're on a fairly wide 30 mph limit road in town. Ahead on the left is a large parked lorry unloading goods into a shop also on the left. There are quite a few pedestrians on the pavement, but for the moment there are no other vehicles ahead, behind, oncoming or parked. As we approach the lorry we assume that someone might be hidden by the lorry and planning to cross the road. We adjust our road position to the right to give us a safe margin from the parked lorry itself and an earlier view into the hidden area. Notice that as we position further right we get a spin-off benefit in that any pedestrians emerging from behind the lorry will be visible to us for some time before they actually come into our path. Once we have this lateral separation it is no longer required to be travelling so slowly that we can stop as we reach the far end of the lorry. However we do slow to under 20 mph and as we get to the rear of the lorry sure enough and right on cue some pedestrians appear from the hidden position. If we have planned safely and correctly we have ample time to stop.

Many less drivers think that you cannot plan for the actions of other road users, and you cannot plan for unseen hazards. Both these ideas are wrong. We saw in the example above that it is quite realistic - even normal - to plan for the unseen hazard. As far as other road users are concerned the same principles apply. Assume the worst until until you have positive confirmation of their behaviour.

Using these skills the advanced driver can avoid the vast majority of potential accidents. It starts with hazard perception, then a safe road position and a safe speed follow. 

Speed limits are dumb

I hope we've shown that the vast majority of drivers use the safe speed rule unconsciously but successfully each and every day. On the road it's the safe speed rule which saves your life and controls the real risks to which you are exposed. Without it we would all be crashing within a few miles of leaving home. On the other hand a skilled driver could drive safely for an entire driving career with no reference whatsoever to the speed limit. You don't need a speed limit to tell you that slowing down in a village high street is required to be safe; you just need to observe the hazards and anticipate the risks. Just to emphasize the point we know of drivers who have driven for extended periods without a working speedometer. And what happens? Absolutely nothing.

So there you have it. Far from the government line about "you must stick to the speed limit to be safe", we all know that we can drive safely completely without reference to numerical speed or the speed limit. Inexperienced drivers without the advantage of good hazard perception and anticipation skills benefit from the guidance that the speed limit provides. Other than that the speed limit is dumb. It fails to take account of actual hazards or conditions and instead gives a useful but vague warning about hazard density.


 
0.45 g stopping distances for normal driving
speed thinking distance braking distance overall stopping distance size of safe braking zone in seconds
20mph 22 feet 30 feet 52 feet 1.8 secs
30mph 33 feet 67 feet 100 feet 2.3 secs
40mph 44 feet 120 feet 164 feet 2.8 secs
50mph 55 feet 187 feet 242 feet 3.3 secs
60mph 66 feet 269 feet 335 feet 3.8 secs
70mph 77 feet 366 feet 443 feet 4.3 secs
80mph 88 feet 478 feet 566 feet 4.8 secs
90mph 99 feet 605 feet 704 feet 5.3 secs
100mph 110 feet 747 feet 857 feet 5.8 secs

 
Notes for the table:

The Highway Code braking distance figures are different from these in two important respects. The Highway Code assumes about 0.68 seconds reaction time and 0.67g braking. Modern cars braking in the dry and on the flat brake at about 0.9g. We recommend never planning to use more than half your maximum braking effort in normal driving which leads us base the above table on 0.45g. The slight difference in the reaction time allowed is simply because we consider 0.75 seconds to be a little more realistic.

The size of the safe braking zone is NOT the time it takes to stop. In fact actually stopping takes around twice as long. However if you look (for example) 3.8 seconds ahead at 60mph you are looking at precisely the point where you will stop if you suddenly decide to brake.

Download our braking physics spreadsheet

It's an Excel '97 format spreadsheet, You can alter speeds, thinking time and braking effort in "g" and see thinking distance, braking distance, overall stopping distance, and the size of the safe braking zone in seconds.

On other included sheets you can see graphs of overall stopping distances and safe braking zone as they change with speed. Click the thumbnail image to download. 32KB.


 
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Copyright © SafeSpeed 2001,2,3,4
Created 2001. Last update 7/03/2004

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