How to get a time cost related to polyline geometric characteristics in postgis using OSM data?

I'm trying to make an isochrone mapping of the emergency facilities in my region, I'm using a street map loaded in postgis by osm2pgrouting, data is supplied by OSM.

The only cost I got for every polyline is the length that is suitable for a iso-distance map not for an isochronic map, so what I miss is the time cost and a good method to calculate it.

The first method I used was the length/maxspeed ratio but the results are unreal (nobody is able to drive at maximum speed all the way long).

So I thought that a good approach could be calculate for every polyline the travel time using only geometric characteristics like length, tortuosity, radius of curvature, etc. (obviously I have not access to any time travel database).

I hope someone has a good solution for my problem. The software I'm using is postgis 2.1, pgrouting, osm2pgrouting and QGIS 2.8 with pgrouting layer plugin.