I go with the 405 over the 5 every time – the 5 has a long stretch that is JUST TWO LANES in both directions at the southern end of LA county – If you want to check online before you go ( or on your PDA if you carry one), visit TANN (http://traffic.tann.net/tanntraffic_trafficmap.php )
Lastly, if Both the 5 and the 405 are hosed, there is another route you can use which MAY be faster depending on how bad the 5 and 405 are – 57 North, 210 West. Typically this is not worth the extra 30 miles, but I have seen crazier things as far as LA traffic go ….
