What I did for making roads in my terrain went something like this:
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When designing my heightmap, I made notes of where the roads (or, more specifically, where the spline paths for vehicles) would need to be. Having terrain that changes elevations really quickly creates a big problem when you’re trying to make your vehicle actually keep all 4 wheels (or whatever) on the ground.
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Once I knew where my roads needed to be, I created the road through the terrain, making i about 3-4 pixels wide the whole way. I used the color representing the lowest height on the road for this first pass.
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Next, I just went and gradually increased/decreased the height as necessary throughout the road I had made, making sure that I made mini gradients when adding hills/depressions to make sure the height change wasn’t soo sudden to cause problems.
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I then went and fixed the terrain pixels around my roads by adding some median colours between the road itself and its surrounding terrain. This was done to make the terrain seem more natural, and not like the road is the product of a rock cut. If that’s the look you’re going for, though, then leave it. Heh.
One thing to note is that 4 pixels might be too wide for your needs/abilities (somtimes you only have so many pixels to play with in an area), so a good measure is to make a small patch of this road, and toss any models you will but putting onto this road onto it in Radiant. That should help you guage the width necessary for your roads. IF you don’t need and vehicles to go on it, of course, then you don’t really have to worry about making soft hills/depressions so much, since the players move a little better than vehicles do.
Also, I’m one of those people who couldn’t figure out how to make a 300x300 image make a map that wasn’t ridiculously large. Instead, I figured out how much 1 pixel equated to in size in Radiant, and made a 28x40 (or something to that extent) heightmap accordingly.
Anyway, as Joop Sloop mentioned, you can just do the terrain by hand if you want. I’d imagine that would take forever, though.
Finally, another thing you want to avoid is modifying your terrain in Radiant where possible. You can only really pull terrain up or down to a certain degree before you get sparklies. And nobody wants sparklies.