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Colin Smale wrote on 01/11/2010 19:32: So why not start documenting all these defaults or implied values? Here's a few suggestions to get the ball rolling. Someone has not been on their speed awareness course, have they? (ahem!) 70mph is based on dual carriageway. A dual carriageway does not need two lanes to qualify, and this is more common these days as you find that on certain dual carriageways a lane has been blanked out for some distance. Also, to nitpick, your implications are not all correct, and lanes >=2 does not imply any sort of maximum speed, as any decent one-way system in a town would match this, and you cannot rely on a trunk road to identify this as truck roads often run through built up areas. In other words, there are sensible defaults, but you cannot imply speed limits as easily as you suggest from the tags you present there. Similarly, although again a sensible default, residential roads with 40mph or more are not uncommon. However, that is pedantic, and I'd agree in principle with defaults being sensible, indeed road signage in the UK is based on the principle that you can infer the speed limit from the presence or absence of street lighting if there are no contradictory speed limit signs (motorways being an explicit exception to the rule). This has saved me from a speed camera incident or two in the past. So to go down a proper mapping exercise to determine actual speed limits, we should be mapping the limits of street lighting as that is the relevant attribute, though to be fair, I can barely recall an example where a council has relied on the presence of street lighting alone to control traffic speed. Changes in legislation a few years ago make understanding this implicit speed limit more relevant, as it is now no longer a requirement to signpost increases in speed limits in certain scenarios, not is it a requirement to have signs either side of the road in all cases and your only clue might well be that you see a repeater sign somewhere up the road - the theory being that when you exit a minor road, you carry on at the same limit until you are informed otherwise (e.g. exiting on an unlit country lane which was 30 mph onto another unlit country lane you might find a repeater sign a bit down the road saying 40mph without an intervening speed limit sign to show the increased speed. So a little bit of care is needed in mapping speed limits to ensure you map based on the subtleties. Of course, these speeds do not apply to vans which are not car based, nor buses and lorries. A surprising number of white van men drive at a licence losing 30mph over their 60mph speed limit on dual carriageways and 50mph on single carriageways; and perhaps you should not curse the lorry on a typical single carriage A road where they are limited to 40mph by law, 50mph on a dual carriageway. It gets more interesting for lorries on a motorway, because although they are allowed to do 60mph in terms of speed limit, lorries now have to have a speed restrictor and digital tachograph that limits them to 56mph to comply with EU legislation so this is the de facto limit. So any maxspeed needs to account for the type of vehicle, (or it should be a code). This is even more the case in Europe where it is common to see sections of autobahn or trunk road where the speed limit is explicitly varied by type of vehicle. FWIW, Tom Tom maps speed changes pretty accurately, though it is not completely reliable, but the competition is pretty high. Spenny (currently pointless in mpre ways than one!) |
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