Generally the maximum speed is handled by computing the travel time of a road based on its specified max speed. So the route solver minimizes the overall travel time. If you have a vehicle that travels at 5 miles per hour even on a 35 MPH road, than you can compute the travel time on all the roads in the network based on 5 miles per hour and then solve the path. Or you could set up a parameterized attribute where at run time you specify the speed of the vehicle and the travel time for each edge is computed at solve time (slow performance). You can read more about parameterized attributes here: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Network_analysis_with_parameterized_at...
