Twelve miles an hour, about 20 km/h, is often touted as the average speed of traffic in London. At best it's no more than a rough guess at how long it might takes to get from one place to another, but it's a good starting point for what follows.
What if there were a 12 mph universal speed limit? Not just in London, but nationally? Or indeed, across Europe? The lower the speed limit, the closer the average speed across a trip is likely to be to the speed limit. For example, a 100 mile journey (160 km) with speed limits varying from 20 mph to 70 mph (roughly 30-110 km/h) is likely to take 2-3 hours, depending on the relative proportions of motorway and urban street: an average of 35-50 mph (60-80 km/h). With a 12 mph limit it would take 8.5 hours if you drove flat out all the way, but you'd be going flat out for much more of the time: you could probably do it in 10 hours quite comfortably.
For any journey of more than about 5-10 miles, call it 8-16 km, there would therefore be a great incentive to use (much faster) public transport. Sometimes I go down to Spain, maybe an 800 mile (1300 km) round trip, and the idea of three days of solid driving each way would greatly discourage me. On the other hand, I really wouldn't mind taking an hour or even two to get to the shops in Loudun or Thouars, provided I could load my groceries into the car. For that matter, motor-rail (drive your car onto a flatbed, or into an enclosed carriage, Channel Tunnel style) would look extremely attractive.
Then again, those who were willing to take a few days to travel 300 miles (500 km) at a maximum of 12 mph would find that they had a great deal more time to appreciate the countryside they traversed, It would also benefit those who cater to the traveller: hotels, guest-houses, restaurants, cafe-bars.
In town, a 12 mph limit brings more numerous and general advantages. Fewer accidents (more reaction time) and less serious injuries if there are accidents (lower impact speeds) are only the most obvious. The vehicles can be made lighter, because they don't have to resist high-speed impacts and because they won't have the jolting and banging caused by fast travel on anything but the very best roads, perfectly maintained, They could be both cheaper and more durable than modern motor-cars: there'd be no enormous incentive to build them as lightly as possible, but equally, at the same cost per pound (or kilo) as current motor vehicles, they'd still be cheaper. Insurance should go down as a result of all this: fewer accidents, less damage, cheaper settlements. Fuel consumption would be far lower, obviously, with associated reductions in energy consumption, global warming and air pollution.
Wear and tear on roads would be much smaller with slower, lighter vehicles, and taxation (both purchase price and annual Road Fund) could be based on axle loading. Road wear is widely held to be proportionate to the fourth power of the axle loading, but let's be generous and base the tax on a cube law instead. With (say) no tax payable below half a ton per axle, and a basic Road Fund of £20 a year for half a ton, then a three-quarter ton axle loading would attract £67.50 (1.5 x 1.5 x 1.5 x 20) while one ton would attract £160 (2 x 2 x 2 x 20) and a 32-ton, 8-axle truck (4 tons per axle) would be taxed at around £10,000. A fourth power law might be too dramatic, at £20/£135/£320 and £80,000. The same ratios are of course equally true for dollars or euros.
Legislation might need to specify the maximum number of axles per ton, in order to avoid an 8-axle 4 ton truck (half a ton per axle, £20 tax), though probably the cost of making a 16-wheeler 4-ton truck would be prohibitive anyway. Over the years, the minimum axle loading that attracted tax could be lowered progressively, preferably with a grandfather clause for older vehicles.
A speed limit of 12 mph also has the happy coincidence of being roughly comparable with the kind of cruising speed that most non-competitive cyclists are likely to achieve, other than downhill with a following wind. There will still be motorists who hate cyclists, but now it is likely to be because the cyclists can go faster, rather than because the motorist is stuck behind them. There is no particular reason to subject bicycles to the speed limit: the low mass of a bicycle means that risks to pedestrians are modest, and because of their vulnerability, cyclists have more of an incentive not to crash than motorists. There can still be an offence of "wanton and furious driving or racing" (Section 35 of the Offences Against the Person Act 1861) and indeed it could even be extended to cases where there is a serious risk of injury, instead of the actual injury required by the Act.
The limit needn't be especially expensive to introduce. Just about any existing vehicle, of any vintage, can be fitted with a speed limiter at quite modest expense: probably modest enough that anyone who could afford a vehicle at all could afford to have one fitted if it were a condition of continuing to use the vehicle. All new cars would have them as standard, while those already in use could have them fitted as a condition of a certificate of roadworthiness, like an MoT test in the UK. To be sure, limiting old vehicles with manual gearboxes to first or second gear would mean that the engines were seldom working at maximum efficiency, but that's still an improvement on being forced to scrap them.
There'd need to be progressive penalties for breaking the limit, perhaps points-based, with an automatic loss of licence at (say) 25 mph or 40 km/h (downhill with a following wind, again), and there would have to be Draconian penalties (including automatic forfeiture of the vehicle) for by-passing or de-activating the speed limiter and then driving on a public road. It might also be possible to arrange for the speed limiter to provide an internal warning, such as a buzzer, for older vehicles passing the speed limit downhill with a following wind, and external warnings such as flashing the headlights and sounding the horn beyond (say) 15 mph. Another possibility is incorporating a tachometer in the limiter, recording only the dates, times and speeds of infractions.
A separate issue entirely is that it might be possible to lower the driving age. After all, you can ride a moped at 16 in the UK, 15 in Denmark, 14 in France, and 12 mph is a lot slower than most mopeds can go. There might however be a case for restricting the size or weight of cars that could be driven by very young drivers.
Finally, the 12 mph limit really comes into its own with driverless vehicles. There's just so much more reaction time available, both for the vehicles and for everyone else, though it's tempting to use "victims" as a synonym for at least a proportion of "everyone else". Up to about 12 mph/20 km/h, everything is on a human scale: many people can just about run at 12 mph, albeit not for long periods. After all, even the famous "four minute mile" is 15 mph, just under 25 km/h, and world-class sprinters can cover 100 metres in under 10 seconds: 600 metres per minute or better than 36 km/h, over 22 mph, This is one of the major reasons for setting a 12 mph limit instead of 20 mph.
The point is, you'd seldom need to run at even 12 mph, and then, only for a few paces, if you felt you were threatened by a self-driving vehicle at 12 mph. Public transport, on segregated roads or railways, could be a lot faster. The "segregated roads" could quite easily be made from existing 4-lane dual carriageways, with private vehicles on one side of what is currently the central reservation, and (much faster) public transport on the other side. Crossings would not be beyond the wit of man: we have, after all, over 200 years of railway experience to draw upon.
Of course, it's all something of a pipe dream. Too many things have to change to make it feasible in the near future. But then, the current model of consumerism, predicated upon perpetual growth, is even more demonstrably infeasible in the long run. As (probably) is banning private vehicles altogether: ask anyone who lives in the country, rather than in a city.
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Words and picture copyright (c) Roger Hicks 2016