From a Technical Journal article I read some time back How we driveResearch by the (US of A) Federal Highway Administration has shown, closing down a highway entirely means the job gets done, on average, 63 to 95 percent faster than projects that tried to maintain a semblance of traditional traffic.
No traffic means no interference from drivers, no work-zone crashes (in 2007, for example, 835 people were killed in work zone crashes) or other bad behavior, not to mention that the trucks hauling materials and workers don’t have to sit in the same congestion as everyone else as they go back and forth.
The secret to making this happen, as is happening in Los Angeles, is to enact a comprehensive “Traffic Management Plan,” with careful study of alternate routes and “network effects.” Implicit in this is to issue a prediction of Nostradamusian direness; to do for weekend driving what Jaws did for ocean swimming (“just when you thought it was safe to go to Santa Monica”).
This reason this generally works is that in any road system, there is a certain amount of elasticity; not every driver on that road has to be there at that time. There may be another route, another mode of travel. Or they just stay home. When highway segments are taken out because of disaster (as in the Minneapolis I-35 W bridge collapse, or the collapse of Manhattan’s West Side Highway) the surrounding roads do not automatically filled up with all the diverted drivers; rather, some traffic “disappears.” To quote two of the main findings of a report analyzing any number of road closures, planned or otherwise, by transport researcher Phil Goodwin and colleagues: “When roadspace for cars is reallocated, trafﬁc problems are usually far less serious than predicted” and “Trafﬁc reduction is partly explained by recognizing that people react to a change in road conditions in much more complex ways than has traditionally been assumed in trafﬁc models.”
When Los Angeles partially closed the 710 expressway for eight weekends, it was able to reduce traffic by 37%. Interestingly, though, traffic was lowest through the work zones the first weekend, and then grew gradually on each successive weekend, as L.A. drivers, in a kind of city-wide learning curve, began testing the drive. In the case of the 405 closure, of course, drivers won’t have that option. There’s no knowing how bad or how good it’s going to be, until you’re in it.