Postgis walk through multilinestring

Question

Maybe question title not correct, i have no idea how to formulate my question.

Database is postgres 9.2, POSTGIS="2.0.2 r10789" GEOS="3.3.6-CAPI-1.7.6" PROJ="Rel. 4.7.1, 23 September 2009" LIBXML="2.7.8"

I have roads map from OpenStreetMap database. Geometry type is MultiLineString. For example, road from city A to city B consists of 27 entries in database (200 km total distance). One entry can be 500 meters another can be 70 kilometers. I want to analyse this road in fallowing scenario:

1. From city A take 40 km in direction to city B.
2. Then analyse that path and in case if i need, go, for example, 5 km back or forward.
3. Take another 40 km, analyse until i reach city B.

How i can go N km from point A to direction B via that road? It is possible with SQL / postgis?

What i need is, plan where to place green fuel station. Electrical vehicle can drive average 50 km with full battery. So i need to analyze every N km if there is possible to build fuel station.
I already have data about rivers, electricity grid and another restrictions.

Thanks.