[Routing] Getting A* to prefer Motorways

[Jon Bright]

Hi Andreas,

Andreas Leupin wrote:
> Hi Marcus, hi Jon,
> even for the shortest route the heuristic isn't correct in every case. 
> For example, if you are on a peninsula with the target on the other side 
> of a bay it may be better to move first away from the target instead of 
> searching directly a way in the direction of the target.

See Frederik's reply - for A*, the heuristic should always be optimistic 
about the distance/time to reach the destination - i.e. it should never 
over-estimate, always under-estimate.  It should therefore give the 
straight-line distance (or, better, the haversine distance) between the 
point it's at and the destination point.

You can have a play with A* using JSearchDemo, which helps with 
understanding the algorithm: http://el-tramo.be/software/jsearchdemo

The closer the heuristic is to the actual distance required, the quicker 
you'll find the optimum route.

--
Jon