Comments on "cars"

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Proposition: Cars and similar vehicles should be required by law to have at the rear a prominently-visible digital readout of their current speed.

Advantage 1: it would be that bit easier to match the speed of the car in front, which is generally a Good Thing for efficient traffic flow. (Actually you might want a low-pass-filtered version of the speed of the car two in front, or something; anyway, whatever it is you want, the extra information will do no harm.)

Advantage 2: it would act as a mild deterrent to speeding.

Advantage 3: it would make it slightly easier for police to catch people speeding.

(What's that you say? Sometimes driving faster than the limit is a perfectly reasonable thing to do, and sometimes the police prioritize fundraising over safety when it comes to catching speeders? Quite possibly. In which case, the thing to do is to fix the laws or the speed limits, not to rely on the fact that sometimes it's inconvenient to tell whether someone's speeding or to prove they are.)

It might be difficult to do this visibly enough to be useful without obscuring the view through the rear window; that might be sufficient reason not to do it.

On 2010-04-19 at 01:13:25, Joey M said:

Advantage 1 could be achieved with some kind of measuring device in the dashboard to measure the speed of the car in front of you. This way never obscures anyone's rear view, and it is unnecessary to retrofit old cars.

On 2010-04-19 at 08:56:33, g said:

Yes, you could do it that way. (Some cars already have the ability to measure where the car in front is and keep an approximately constant distance from it; I don't think they report its speed, but there's no reason why they couldn't.)

But I think advantages 2 and 3 are real advantages too.

Oh, and I wasn't proposing (in so far as I was proposing anything) that old cars be retrofitted. If, as I suspect, many drivers would prefer not to have the new devices in their cars, leaving old ones unaltered would have the effect of encouraging people to drive older cars for longer, which would be good for the environment (older cars are environmentally worse, but making new cars is so bad for the environment that it outweighs that) but bad for the economy. I don't know whether the tradeoff would be a good one.

On 2010-04-19 at 09:24:08, Clive said:

Problem: car speedometers aren't accurate enough to use in speed matching with the vehicle in front, and can't easily be made that accurate. The difficulty is that a car tyre changes radius by 4% over its lifetime (even if consistently kept at the correct pressure) and the best thing a speedo can measure is the angular velocity of the final drive shaft from the gearbox. GPS is more accurate over distance, but not even slightly accurate instantaneously.

The police VASCAR system for matching speed with a driver then reading off the police car's speedo requires daily recalibration. (People can and have had convictions quashed because the police couldn't produce that day's calibration record.)

More generally, we have hard and fast laws that are prosecuted vigorously and we have more malleable laws where enforcement is more lax. It's a mistake to drift towards lax enforcement of the former or to demand a zero-tolerance policy on the latter. Personally, I quite like the status quo, where it is well understood that outside urban areas you always go to court if found doing more than 30mph above the speed limit, sometimes go to court 25mph over, normally get an on-the-spot fine at 15mph over and will usually be ignored if going slower than that. It's good that this is only an informal understanding: it means the police have the discretion to let people off with a Stern Talking-To for 95mph on the A1(M) at 3am, but can pull someone over for 62mph on a single carriageway if they're behaving like an arse.

I don't think the problem is other drivers not knowing what speed someone's doing. I can tell at a glance; I expect most other experienced drivers can, too. What's more, they probably know I know, and they don't care.

There were trials a few years ago of speed readouts in roadworks, which alternated a readout of the car's speed with a smiley or frowny face as appropriate. They've fallen out of favour because of the number of idiots wondering if they had a hundreds digit and other such antics. Similarly, a lot of people would quite like their mate to follow them and get a cameraphone photo of the 130mph readout in the back of their car down Trumpington High Street or whatever.

On 2010-04-19 at 19:54:24, g said:

I bet you could make a very accurate speedometer on the same principle as the optical mouse. (Keeping the optical path clean enough might be tricky, but it doesn't seem at all impossible.) Or you could stick with axle motion and calibrate against GPS readings. I simply do not believe that it's not possible to make an accurate speedometer if you actually want to do so. However, I appreciate that if present-day speedometers are inaccurate then that reduces the utility of a system like the one I proposed.

I think it's entirely possible that it's best for speeding to be treated laxly. Making it easier to enforce speed limits more vigorously is not the same as making it compulsory to do so, and if the Right Thing is for the police to ignore many instances of speeding then they can perfectly well continue to do so even if the evidence is staring them in the face. I'd welcome the greater transparency.

I take your point about perverse incentives. There's a similar objection to something else I've always half-wanted (rather less than half, actually), namely a "sorry" light on cars. Used properly, that might have some value for defusing annoyance on the roads, but in practice you'd get (1) people thinking it's OK to do obnoxious things provided they "apologize" and (2) people deliberately doing super-obnoxious things and then using the sorry-light sarcastically. Fail. Anyway, my guess (which is no more than that) is that the benefits would outweigh the harms, not least because you'd need two stupid drivers rather than one. And, after all, you've already got a speedometer inside the car.

On 2011-06-23 at 01:12:16, Danny McCaughan said:

The first disadvantage of the idea is that a likely consequence is distraction of the driver following the vehicle with the (continually changing) display. The more prominent it is the worse that problem becomes, and the higher probability of accidents. The second disadvantage is how big it would have to be to fulfil the stated criteria. A requirement of the driving test is that the person being tested be able to read a number plate at 20 metres with letters 79mm high. As the rear display would have to show up to 4 digits with a decimal point it would in effect have to be the size of a number plate and be brightly illuminated to be readable at a practical distance by a person with marginally legal eyesight. The distraction effect even in the daytime would therefore be significant, potentially causing accidents. The third disadvantage is that at night-time it could add confusion especially in fog, leading to another route to accidents, especially on a vehicle with a faulty rear light. Fourth - Where would such a large display go on a motorcycle? As you can see I do not think this idea is practical!

On 2011-06-24 at 00:05:51, g said:

Good point about distraction, but I find some of your details unconvincing.

I don't think the thing would need to show "up to 4 digits with a decimal point" -- whyever would it need resolution better than 1mph or 1kph? Displaying in mph, I think 2 digits would be enough. (No problem if it just shows "99" for three-digit speeds.)

A vehicle with a faulty rear light is highly dangerous in fog as it is. Having an extra (known) source of light on such a vehicle seems to me at least as likely to make things better as to make them worse.

Motorcycles could be exempt.

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