There's nothing in the API that will help with that. You'd have to write
your own direction finder.
A bus route direction finder is easier to write than a general purpose
route finder because you have a much smaller set of nodes, the bus
stops, where the route can change. If your city isn't too large, it's
feasible to perform a brute force search.
Allocate a "cost" to travelling each segment of bus route, from one stop
to the next. GDirections uses estimated elapsed time as the cost, but
you can use whatever you want to minimise. In particular, it would make
sense to add a high cost factor to changing busses, so that your system
would prefer a longer route with one change of bus to a shorter route
which involved two changes. It would also make sense to include the
frequency of the busses on the route, so that your system would prefer a
longer route that involved a bus that runs every 10 minutes to a shorter
route where there's only one bus per hour.
Once you've created your "cost" metric, and assigned costs to each
segment, assign a cost of zero to the destination node and Infinity to
all the others.
Repeat:
Find the unprocessed node with the lowest cost, call it node A.
Loop through all the nodes (B) that are one segment away from node A.
Calculate their costs via this route by adding the cost of the segment
to the cost of node A.
If this cost is less than the previous cost assigned to node B, assign
node B that cost, and record that the route goes via node A.
Mark node A as processed.
Until: the start node is the unprocessed node with the lowest cost.
At that point you have the route from start to destination that gives
the lowest cost.
--
Mike Williams
http://econym.org.uk/gmap
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