[Talk-GB] Bridge height - speed limits

[Simon Ward]

On Sat, Jun 06, 2009 at 09:34:32PM +0100, Richard Bullock wrote:
> Of course the actual true height of the bridge does convert.
> 
> 1 foot is exactly equal to 0.3048m - by definition

Forgive me if I don’t go around measuring the true height of each
bridge.  (That’s before we come to arched bridges.)

Yes, the heights are advisory, but both are useful and I feel both
should be tagged.  I don’t believe I should be the one making the
decision on how to deal with dual signage; I should be gathering the
data that’s there and leaving it up to the data users to work out what
to do with it.

> Speed limits on the other hand are a limit. In a black and white version of 
> the law, you could be prosecuted for exceeding the speed limit by any 
> amount. There's no safety margin akin to the bridge heights.
> 
> 30mph = 48.28032 km/h exactly. There's no rounding in that conversion. The 
> mile is defined as exactly 1.609344 km

I realise this started with how to tag in the UK, and is still on the
talk-gb list, but the UK isn’t the only place being mapped, and I’m open
to other places having dual signage.  If these Wikipedia pages[1][2] are
anything to go by, some places do, and both limits are rounded:

    “Houston, Texas has some signs in both SI and imperial units near
    its airports and downtown.”

In addition, not all speed limits are created equal[3].

[1]: http://en.wikipedia.org/wiki/Speed_limit#Units_of_measurement
[2]: http://en.wikipedia.org/wiki/Speed_limits_in_the_United_States#Metric_speed_limits
[3]: http://www.mit.edu/~jfc/laws.html#types

Now, I haven’t surveyed any of these, but if I did it would be wrong of
me to use one value in favour of the other without more knowledge of the
legalities.  This is the crux of my original suggestion.

> We should therefore:
> 
> Use maxspeed=30 mph. (It should be trivial for any pre-processing of data to 
> get this in the right format for whatever application it is required)
> *or* use maxspeed=48.28032
> 
> maxspeed=48 is wrong - you are legally allowed to travel faster - so hence 
> it is not a maxspeed. Maxspeed=48.3 is also wrong - as the true maxspeed is 
> lower than that. 

It would be even more wrong of me to attempt to convert, potentially
losing accuracy.  I never said do that.

In summary, I say there should be a scheme that allows to tag dual
signage where it exists, as well as cope with single signage in
different units.  Don’t approximate, ever.

Simon
-- 
A complex system that works is invariably found to have evolved from a
simple system that works.—John Gall
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