You need to know all area coordinates.. so you need to create imagemaps manually.. you can not tell PHP to 'look for asphalt' and let it work as an area...
so create image maps for each circuit.. and name them the same as your circuits...
<img src="image_circuit.php?circuit_id='.$id.'" usemap=circuitmap$id>
<map circuitmap1>
<!-- define areas -->
</map>
<map circuitmap2>
<!-- define areas -->
</map>
something like that...
even better would be, creating include files of them, so you don't have HTML code, not used in your source.
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