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When I connect a trunk road to the network, the routing software will sometimes suggest an an illegal and dangerous u-turn maneuver. How do I fix it ? |
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First, that's not a U-turn but a sharp left turn. Second, the question is about a single-carriageway road with motorway-style ramps, which may include a two-lane motorway. You'll need to split the trunk road at the location of the illegal turn and then add a turn restriction, either a no_left_turn from the ramp to the trunk, or an only_right_turn from the ramp to the other segment of the trunk. (Note that for an offramp, you can only use a no_left_turn, not an only_right_turn, since the latter would require traffic to exit and would not prohibit the left turn.) You'll also want to add a no_u_turn restriction between the two carriageways if a dual-carriageway portion becomes single-carriageway. For an example, see SR 570 in Florida. In a left-hand-driving country, reverse the directions above. I think your interpretation of only_* is wrong. The wiki clearly states that all traffic on the "from" way that reaches "via" must continue on the "to" way, and that interpretation has been fixed for over a year.
(17 Jul '10, 00:35)
Nic Roets
In your example, you can only turn right from Kelso to US 26. So an only_right_turn would be correct with from as the ramp, via as the end node of the ramp, and to as US 26 to the southeast. no_left_turn would have to as US 26 to the northwest.
(17 Jul '10, 01:25)
NE2
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Something seems to be wrong with the router here IMHO. It doesn't use the first, obviously shorter off-ramp and uses the the second off-ramp against the (implied) one way direction.
RoToRa, this is in the US, where we drive on the right.
I'm aware of that :-) Here's what I get: http://img687.imageshack.us/img687/6264/unbenanntxl.gif IMHO it should route along the off-ramp with I marked in red.
RoToRa: The direction of travel is the other way, i.e. travelling east from the tertiary road, then going northwest onto the trunk road.
D'oh. Ignore everything I said.