|
 previous / next column
A PHYSICIST WRITES . . .
(November 2006)
The summer issue of the Advanced Driving magazine contained a fascinating survey of the ten editions of the Highway Code that have been published since it was launched in 1931. One surprising fact stood out among many: the Table of Typical Stopping Distances is unchanged from when it first appeared in 1946! Apart, that is, from metres being introduced and the table being extended from 50 up to 70 mph. Have there been no improvements at all in brakes, tyres or tarmac since the war? Not in the eyes of the Department for Transport.
In my September 2005 column I cast a physicist’s eye over the Table. The Thinking Distances are calculated on the assumption that it will take you about 2/3 of a second to react to a hazard by hitting the brake pedal. The Braking Distances assume that you will then slow down by 15 mph each second, until you stop. Near the table is the advice that the “safe rule” is to leave a space equal to the total Stopping Distance between you and the vehicle in front. You are also told to allow (at least) a two-second time-gap, in fast traffic. But at 70 mph, the Stopping Distance rule means keeping at least a three-second gap, as I pointed out last year. Has no-one at the DfT noticed this discrepancy?
The magazine also revealed some of the new advice likely to be put into the 2007 edition of the Code. I’ve read elsewhere that this will appear in the spring — and that the Table of Stopping Distances will be revised at last. I imagine the compilers are tearing their hair out over these! They cannot possibly continue to pretend that cars are unable to brake more efficiently than 60 years ago. On the other hand, reducing the Stopping Distances would in effect be telling everyone that it’s safe to drive closer to the next vehicle than they do already.
Let’s see if I can assist them in their dilemma. As I said, the current Braking Distances are based on losing 15 mph per second. But when I took my Corolla out for a spin in good conditions on an empty road, I found I could brake safely from 45 mph to a halt in 2.4 seconds, rather than the 3 seconds expected. From this I calculate that all the Braking Distances in the table can be cut by 20% and still remain ‘typical’ for a good road.
On the other hand, I think the Thinking Distances are too short! Two-thirds of a second may seem time enough for the brain first to register a hazard that your eyes have seen, and then to decide what to do about it. But only after this will your foot start to move across from one pedal to the other — another fraction of a second gone. And don’t forget you could easily be glancing away at the vital moment, to the side of the road or down at the instruments (or over your shoulder, if you are a TV reporter reporting as you drive). So I would argue that the so-called Thinking Distances should correspond to at least a second and a half of travel. This means multiplying them all by two and a quarter.
Putting these two changes together, the overall Stopping Distance at 30 mph increases from 23 metres to 31, while at 70 mph it goes up from 96 metres to 107. So while demonstrating that brakes have become more effective since 1946, I have still managed to justify an increase in Stopping Distance. I can hardly wait to see whether the authorities will have the courage and the common sense to do the same in the new Highway Code, when it comes out!
I wonder too if they will make it clear that a time-gap such as two seconds that’s safe at a low speed may not be sufficient at a higher speed. And will they mention, as I did last year, that your braking capacity (ie, how hard you can brake without skidding) may be significantly reduced when you’re driving round a bend or travelling downhill, ABS or no ABS?
Leafing through the current Code, I notice one or two other items that could do with clarification. “Never sound your horn aggressively,” it says. But I’ve owned very few cars in which you could toot politely (ie, just ‘bipping’) except with much practice and concentration. Then at mini-roundabouts we are told to “beware of vehicles making U-turns.” An instruction to the culprits not even to think of making U-turns might be more beneficial to road safety. Another warning is: “Do not drive in icy or snowy weather unless your journey is essential.” There’s no mention at all of thick fog, so that’s perfectly safe then. Also, you should not park within ten metres of a junction — but is this to be measured from the stop line (if marked), or from the point back where the kerb starts to curve round? I think we should be told.
Another rule brings back a distant memory: on a motorway hard shoulder, “do not attempt even simple repairs.” Well ... twenty-something years ago my car conked out on the M4. From previous experience, not only did I guess what had failed but also I was carrying a spare. The item was the ignition capacitor, and within a few minutes I was safely on my way. What I should have done, I now realize, was walk in some danger up to half a mile to a phone box and back, wait an hour or more in discomfort on the grass, and then when the flashing light arrived (adding to the distracting of traffic) hand over the capacitor to be fitted! Or else wait to see if the expert could solve the problem on his own.
Anyway, when the new Highway Code comes out I confidently expect it to inspire a further column or two.
Peter Soul
previous / next column
|
