There are a lot of factors to consider. Is this really a "highway," or just a rural road? What era do you model, and how much space do you have to devote to this roadway?
For my HO gauge railroad set sometime between the 1930s and 1960s, depending on my mood, I use 3 inches as the overall width of most roads. I have a couple of 2 1/2 inch roads in an urban area, which worked out well to accentuate the height of the adjacent buildings and give that "urban canyon" look, even with structures only 2 or 3 stories tall.
In realitiy, these roads would seem very narrow, and no one would be able to park along them without obstructing the travel lanes. But, for the sake of modelling, they work out well for me.
It takes an iron man to play with a toy iron horse.
Generally, if we are talking a secondary highway, say the main route between two towns of 15,000 people about 15 miles apart, I agree with Mr. B. Each lane would be about 1 - 1.25" wide, plus about 1/4" or maybe 5/16" for each gravel shoulder or paved shoulder. HO trucks would be wider than HO cars, typically, and they all use the same highway, so something along those lines should be fine.
10-12 ft for each lane, 8 ft for a paved or gravel shoulder and then as much as you want between the shoulder and the clearance line of the tracks for signs.
Dave H. Painted side goes up. My website : wnbranch.com
Best way I've ever found is to just eye ball it. Take a couple of vehicles place then next to each other side by side on a piece of paper and scratch the lines out for the center line shoulder etc. Just take a drive around where you live and observe the roads. DO NOT ge tout in the middle of the road with a tape measure, use one of those digital tape measures you get from Lowes or Home Depot and home the State Trooper doesn't lock you up for being a nutball like he threatened to do to a friend of mine until he found out the trooper was my neighbor and he just shook his head and got back into his cruiser and drove off.
The local US highway is 44' wide since they added the latest emergency strips. However, on my layout, highways are 28' wide. That doesn't leave much passing room for two 14' trailers, but it "looks better" than a wider road. I have a 4' gravel shoulder on each side.