There are really only two ways to reduce the volume of road noise in a vehicle: mass loading panels and adding barriers.

Mass loading is exactly what it sounds like. You add mass to a panel to lower it's resonant frequency. [physics] Everything vibrates best at a specific frequency, but will still vibrate at every frequency to some degree. The idea behind mass loading is that by adding the extra mass, you make it vibrate at a lower period (frequency) to the point where it drops out of the audible range. For example, if you have a fixed energy input (road noise) and a fixed sound level at that energy level, when you add mass to the panel, you are are making it so that it now requires more energy to maintain the same level of output. Since it can't do that (fixed energy input) the noise level must go down. [/physics] So long story short, the heavier a panel is, the more energy it takes to move it. Since the level of energy is fixed, the noise level goes down.

Barriers are exactly like what you think they'd be. They prevent the sound from traveling from one location to another. This is done by combining methods of absorption and deflection. Usually the easiest way to absorb sound is to use a medium that can convert vibration into low level heat. Most closed cell foams are very good at this. (open cell foams are not as effective and often absorb water, causing all sorts of other problems) The best way to deflect the sound is to use a material that is stiff and dense so it won't allow sound to cause it to vibrate. The best barrier materials have a combination of both attributes in that they have a layer of foam with a layer of dense vinyl backing. This absorbs the brunt of the sound in the foam and what isn't absorbed gets reflected back into the foam by the dense vinyl so that the foam gets another shot at it.

Mass loading is particularly effective against lower frequencies as they are the highest energy and easiest to move out of the audible spectrum. Barriers and absorbers work much better against the higher frequencies in the audible spectrum.

There are several materials that attempt to do a lot of jobs at once. A good example is Dynamat Extreme and other styrene and rubber mat materials (RaamMat, FatMat, eDead, etc...). They use a mass loader like rubber impregnated asphalt and an absorber like styrene monomer to balance the properties of both. They act in conjunction as an absorbing mass loader and they are very popular for that reason. Don't be fooled into thinking that the aluminum foil is a barrier though... it's a constraining layer only (ie it's what the material is sprayed onto to give it a shape). Yes it does give it a certain degree of heat resistance but that's more of a side effect... it could be sprayed on wax paper and rolled up just as well

In my not so humble opinion it is always best to use a combination of materials to achieve the best sound deadening. A layer of mass loading material to take out the lows and a barrier/absorber to take out the mids and highs give a well rounded solution to your deadening quandary.

Just make sure when you stuff the insulation that you don't put it anywhere that it might get wet. If it can hold moisture, it will. If it holds moisture, it will mold and cause your panels to rust out. No one wants a stinky car with giant body cancer spots on it If you want a good substitute to that foam, look up DapTex at the Home Depot or Lowes. It is a minimally expanding (non-aggressive) latex foam. It has good cavity filling properties and cleans up with a wet rag (unlike Great Stuff which will stick to anything and everything and expand so much that it can buckle your sheet metal). It also will repel water once cured.