car: better speeds on links

lower speed of links to more realistic levels. keep in mind that the speeds in the profile are average speeds, not max speeds. a motorway_link now has average of 45, instead of 75, similar pattern for other ways types.

also add tests to verify that links are not used unless the main road has a very low maxspeed (about half the default average).

also increased the max to ave factor from 0.66 to 0.8.