Driving Manhattan Beach to Santa Monica, I tried many many different routes to avoid the 405. Turns out, none of them were significantly faster. 45 to 75 minutes guaranteed. All Waze and Google maps did was direct me along obnoxious circuitous routes that got me stuck at too many stop signs. Of course, it took me nearly 3 months of trying to reach that conclusion: statistically tracking peak traffic times, determining when traffic was worst on which streets. So imagine if every driver was trying this, which they obviously are.
I certainly wouldn't have been driving at all if there was a metro line that didn't take 2-3 hours to complete the same route. Though to take the metro from Manhattan Beach it's the Green line to the Blue line to the Expo line and then walk or take the bus another half mile to the office. Or it's take the bus to the LAX and then another bus up Sepulveda and then another bus along Wilshire.
Eventually I just decided to ride my bike because it took just as long and I got to ride on the beach every day. LA traffic sucks enough that people would take public transit if it meant you didn't have to drive the freeways. I will certainly use proximity to a metro line to influence the next place I live.