[Talk-us] Prima Facie Speed Limits

stevea steveaOSM at softworkers.com
Tue Sep 9 17:31:21 UTC 2014

>>I wonder if 15 mph in a school zone and 25 mph in a residential 
>>area are some sort of federal standard?  The source tag might be 
>>useful but not much different than other states.

The federal government doesn't have anything to say about speed 
limits (in states), as the US Constitution leaves such things to the 
states.  An exception is on federal land, such as national parks or 
BLM land, where specific parts of the US Code regarding speed limits 
DO apply.  This is why you can get a speeding or parking ticket at 
Yosemite, but you won't deal with California to fight it or pay the 
fine:  Yosemite is not part of California.  Surrounded by it, yes. 
Part of it, no.

My GPS (a still-tough Garmin 60 CSx) does an excellent job of 
calculating Estimated Time of Arrival (ETA) when routing.  It does so 
by assuming speed limits for all road segments in the current route 
and guesses I'll travel at those speeds.  This is done without 
assigning a speed limit to each and every road segment, instead 
implementing a simple table lookup (freeway:  assume 65 MPH, 
residential:  assume 25 MPH...).  This is a highly efficient solution 
that keeps the data light and the calculation short and simple, so it 
quickly produces an accurate ETA result.

Though I don't find incorrect (or offensive) the suggestion of 
tagging prima facie limits with the tagging syntax specified, I find 
speed limit tagging to be most useful where there are posted signs 
with a specific number.  That's just me, though.

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