In Wisconsin, we use <A target="_blank" HREF=http://www.diggershotline.com/index.html>Diggers Hotline</A>. As some of the other posts have mentioned, the allowable clearances are 18" on either side of the marked locates. But don't bet your life on those!
We've had a number of situations where the marked locates were 24" OR MORE off from the actual location of the "utility"/w3tcompact/icons/shocked.gif. This, obviously, creates some problems when the backhoe slices right through the telephone cable, cable TV, or gas lines /w3tcompact/icons/sad.gif.
Needless to say, when the damaged utility comes looking for someone to pay for the repairs, a whole lot of fingers start pointing in different directions. That's why our crews have started taking pictures of the locates in the areas where they will be working. It's a sad situation, but we automatically assume the locates are wrong before we start. If they're not wrong, great! No harm, no foul. If they are wrong, we've got the documentation to support us when we point our fingers at Diggers Hotline. And if the locates are correct, but our crews are wrong, then they get a visit from a VERY irritated Risk Manager! /w3tcompact/icons/wink.gif/w3tcompact/icons/laugh.gif