The only nasty (venomous) spiders indigenous to western US are black widow, brown widow and (rarely) brown recluse.
All have very disorganized webs and are distinctive - long legs, bulbous abdomen - and surprisingly, not likely to bite unless trapped in clothing, shoes etc. All prefer decomposing timber to car undercarriages (woodpiles and garden sheds are their human created habitat)
Try to avoid being bitten, if only for the fact that they produce tissue-necrosing venom. The bite and subsequent decomposing wound often take months and heavy doses of broad-spectrum anti-biotics to properly heal. I'm no expert - I just got bitten by one. Not at all funny.
