shortcuts behave different than base edges in transition costing

[nilsnolde]

since #2651, we use OSRMCarTurnDuration to add transition cost for traffic light and turns. but we only add this to the Cost::secs in e.g. AutoCost::TransitionCost while we add our "own" logic with stopimpact to Cost::cost.

the problem with that is that when we create shortcuts we can only take into account duration penalties for turn costs, but not the stopimpact logic. that is because we can't set an edge's cost directly, we do it via its speed and attributes.

eventually this leads to different paths depending whether the expansion settled the base edges or the shortcut. I realized that during #4671 which had a lot of geometry differences where it turned out it was because a* expands less and hence uses less shortcuts compared to dijkstra.

it's just something to be aware of. the only thing we could do is remove OSRMCarTurnDuration again and use the stopimpact logic for everything. it could be done to shortcuts then too and likely we'd see more consistent path finding across algorithms.

[dnesbitt61]

I support this. I am not sure OSRMCarTurnDuration is "better" than how Valhalla computes transition costs. Do we have users who depend on this? We use Valhalla transition cost everywhere else - so why would we not use it when generating shortcuts?

[nilsnolde]

Do we have users who depend on this?

sounds like a mapbox thing. so I'd say no.

@kevinkreiser suggested offline to fix our implementation and have only one, i.e. remove OSRMCarTurnDuration again. seems like that's better (less optimistic) at estimating true transition cost from turns/traffic light.

[kevinkreiser]

yeah the reason it was added was because our turn cost implementation was too optimistic which leads to unrealistic ETAs. we certainly care about having realistic ETAs. it would be best if we could amend our turn costs to be similarly pessimistic. doign so properly will affect routes not just ETAs. this hack currently decouples the two so we can keep our routes sane but get worse ETAs.