I guess it depends on where the highway is located and what the prototype roads are like in that area. Roads can be made out of lots of different materials. A forum search using "highways" will bring up several threads and blogs that discuss roads. The search field is in the upper right of the page.

Hope this helps.

Roy

Styrene sheet stock is very popular for roads.
Check out Jas's thread about the JL&T module here: http://model-railroad-hobbyist.com/node/3225

Jappe uses Gatorboard here: http://model-railroad-hobbyist.com/node/3284

I use cardboard for the road base to raise it off the ground a bit.  The cardboard width is 1/4" to 1/2" wider than the finished roadway.  Spackle or drywall mud is used to blend the edges to the other scenery.

The roadway itself is cheap linoleum from a Home Depot type store.  The back of the linoleum is the road surface so it doesn't matter what the "good" side looks like.  Linoleum is much easier to cut than styrene and it is ever so much more flexible if you have hilly terrain.  I glue it down with tacky glue like Woodland Scenics Foam Tack.  Spackle can be used to blend the edges although I just use real dirt along the edges as a transition. 

To make sure my final road color goes on smoothly, I prime the surface with Kilz-2 latex primer.  it has a nice tooth that takes paint well (it works nicely on styrene also).  

I used some of the stuff by Woodland Scenics for making roads.(Smooth It I think) Plaster a paris would work I think. I took some cork road bed and put it at the sides upside down with the angled side in. Pour the plaster in and work it in the sides. I painted it a gray color and left it kind of rough then painted black lines to look like tar to fill in cracks. I even patched a spot. It looks like a old country road that has some age on it. The lines down in front are to big. I did those with a paint brush. The ones in the back I did with a marker, and they are much better. The back ground has been painted since this picture, and the crossings have been worked on.

Alan

I use poster board to make roads.

You can look at how I do it here:

http://www.gamrailroad.com/building/2012/may_28.html

My roads are made to look like concrete but asphalt is just a different color.

Striping from Charlie Comstock, 2010 Nov/Dec issue:

http://model-railroad-hobbyist.com/magazine/mrh-2010-NovDec/utc-model_a_creek_3

Roadwork from Charlie Comstock, 2010 May/Jun issue:

http://model-railroad-hobbyist.com/magazine/mrh-2010-MayJun/utc-roadwork

Web log from Kevin (caboose14) "City Streets":

http://model-railroad-hobbyist.com/node/4771