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 ?

asked 16 Jul '10, 21:10

Nic%20Roets's gravatar image

Nic Roets
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accept rate: 6%

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.

(19 Jul '10, 16:24) RoToRa

RoToRa, this is in the US, where we drive on the right.

(19 Jul '10, 20:50) NE2

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.

(21 Jul '10, 11:21) RoToRa

RoToRa: The direction of travel is the other way, i.e. travelling east from the tertiary road, then going northwest onto the trunk road.

(21 Jul '10, 12:26) Sven L

D'oh. Ignore everything I said.

(21 Jul '10, 15:05) RoToRa

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.

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answered 16 Jul '10, 23:02

NE2's gravatar image

NE2
1.3k333953
accept rate: 9%

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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question asked: 16 Jul '10, 21:10

question was seen: 4,291 times

last updated: 21 Jul '10, 15:05

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