Hi, is height map good for racing game? from what I know, height map is mainly for outdoor landscape, is that right? In racing game there will be tunnels, or sometimes race under a certain structures(bridge, freeway..etc.) can it still be done with height map?? and where I can find informations/tutorials about height map or other techniques suitable for racing game??

As you''ll know a height-map is basically like a tiled surface (though no reason for tiles to be same shape or size) Except each vertex can be lifted of the ground. This is good for must outdoor type stuff but has the problem you can only have one height for the ground at any point e.g you caouldn''t normally have a cliff which overhangs, or a cave, or tunnels etc. However if most of level is outside you could still adapt it slightly:
1)Have the tunnels as a 3d model put on top of the land. A bit dodgy but feasible I think...
2)Use portals e.g a heightmap which would be the highest level for the map e.g over the tops of the tunnels, then have a portal into a long thin room (along the tunnel) which is positioned underneath the surface. Pretty sure that would work in fact it must have been used tonnes of times in games where you go outside then have to do indoors FPS type stuff.

Hope this helps