Low speeds for vehicles (ignoring the very good idea of variable speeds for vehicles) is also realistic because we're talking about 1930s and 1940s era technology.
Back then, 60 mph/100 kph was considered to be dangerously fast for anything other than a race car, both because of vehicle and road designs. High speed roads, and cars which can take advantage of them, are very much post-war inventions.
Another issue is that in a combat zone, often you want to be driving slower than you normally would so that you don't drive into an ambush, or run over a mine. Even in rear areas, partisans would sometimes lay mines or stretch piano wire across the road in order to decapitate people riding in open vehicles.
All that said, being able to set vehicle speeds would be great. Even better would be for vehicles to have slightly better AI. Currently, soft vehicles stop and the crew bails out. In many cases, it's more realistic for vehicle crew to engage attacking aircraft with machine guns mounted on the vehicle, or for small single vehicles, like jeeps or motorcycles, to hit the accelerator and run for cover.