The Mulholland Drive Bridge over the 405 Freeway is scheduled to be partially demolished next month. Authorities have warned drivers that a 53-hour planned closure of the freeway at the Sepulveda Pass will cause significant delays. (Gary Friedman / Los Angeles Times / June 6, 2011)
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Now, if you don't know LA, I can see how you might be incredulous. But this is the road from the west side to the valley.
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The alternate route to the west is 30+ miles of 2 lane coast road (PCH - always backed up & Malibu Canyon Road - almost always backed up).
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The alternate routes to the east are via downtown LA (110 -> 11- >5 -> 134) or surface streets through Hollywood.
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Really, take vacation days and avoid the valley. (Although many people might say "avoid the valley" always, but I digress.)