[OSM-talk] NoName

[David Earl]

On 14/09/2008 23:58, Ed Loach wrote:
> In the UK if you pass a speed limit sign it applies until you see
> another, and you are likely to get most of a town at 30mph with
> signs only around the outside. 

Partly true. It is certainly how the 30mph urban limit is perceived on 
the ground, and in practice lower speed limits are based on areas 
(except things like motorway roadworks) with occasional exceptions for a 
few roads (higher or lower), so it is true that the speed will vary by 
route if you take or don't take one of the exceptions, but not by simply 
going the other way, for example.

Just as there is a default speed limit outside urban areas, in built-up 
areas the default 30mph is signed on entry and not otherwise. BUT all 
exceptions are both signed on entry and exit AND by repeater signs at 
fairly short intervals, reflecting this is an exception.

I have not tagged speed limits except where there are exceptions. All my 
highway=residential or abutters=* are 30mph and the others 60 or 70mph 
depending on single or dual, except where I have put something there. It 
shouldn't be necessary to tag the default speed limit in the UK, IMO.

A good journey time planner wouldn't work just on the speed limit 
either. It is obvious, I hope, that it's a maximum and it is rare to be 
able to maintain it in most urban areas, or even some fixed proportion 
of the speed limit. Even if you account for stops at traffic lights, the 
speed limit isn't a good indicator of what speed you could do in 
practice, and it will vary considerably by time of day and by place - 
you'd get a radically different answer in central London from my village 
even though both have 30mph limits. Of course, it the speed limit is 
*one* factor.

David

PS This is a real speed limit sign in the new town of Cambourne:
http://www.camcycle.org.uk/newsletters/63/images/Cambourne5.jpg
(http://www.openstreetmap.org/?lat=52.2172&lon=-0.0657&zoom=11&layers=B000FTFTT&mlat=52.21841&mlon=-0.07034)