Dynamat and the like are used to change the RESONANT FREQUENCIES of panels which vibrate at their individual resonant frequencies. You will notice that a different panel vibrates on your car with each change of frequency. The most effective way to combat "road noise" is the OEM standard felt carpet backing. (looks like lint rolled out flat). This dense and lightweight material is researched in depth by the auto manufacturers. Try to overlap the pieces if you are unable to find a whole piece for your particular vehicle. Some higher end vehicles have rubber topped felt which helps add weight to the panels as well, further dampening the panels and helping blanket the road noise out.
This only covers the floorboards, however. Doors, roof, and glass are also ways ambient road noise can enter the vehicle. doors are hollow and amplify the road noise. My suggestion is to concentrate on a good door seal, and floor felt, and less on the more expensive Dynamat if you are looking primarily to cancel road noise.