fit point/polyline onto nearest street

Hi all.

I want to simulate a bus network in a network simulation software
(called SUMO).
For the placement of bus stops I plan to use google maps/earth to ease
up the
process and have a nice GUI. That way I get some placemarks.
I would need to "convert" that data into the SUMO format (XML). My
current idea is to
convert coordinates and than compute which is the nearest street and
place the
bus stop at that street.
But as coordinates are not always absolutely correct, this might lead
to errors.
A better idea would be to have the placemarks alongside a polyline
(=bus line)
and try to match this polyline with the according streets in sumos XML-
network.

See this example on how to get the polyline:
http://www.econym.demon.co.uk/googlemaps/examples/snappathdrag.htm
(klick somewhere, it will put a placemark, klick somewhere else it
will draw a
polyline alongside the shortest route to get there).

Do you have any ideas on how to fit that polyline to the existing XML
network and convert
the placemarks to bus stations?

Any help is appreciated.

Gerrit