A lot of good information in this thread.

Rattlers can strike about 2/3rds of their body length unless they're coiled up. When they're coiled up they can strike three times their body length. It's hard to figure how long they are when they're coiled up, so give them a super-wide berth.

Don't put any part of your body into any place you can't see. Check your shoes and clothes carefully. Make sure there isn't the slightest path for them to enter a warmer place at night, like your tent or your sleeping bag.

I strongly recommend an avoidance strategy with any snake not positively identified as harmless.