On April 9, the Tokyo Metropolitan Police Department released pamphlets describing traffic restrictions that would be put into effect if a massive earthquake (Intensity Lower 6 or greater) were to strike the Tokyo Metropolis. The restrictions were made public in Japanese some time ago, but now the pamphlets are available as PDF files in English, Korean and Chinese.
In the event of a massive earthquake in the metro area, TMPD would enforce an initial round of traffic restrictions immediately following the earthquake followed by secondary restrictions once they confirmed the extent and nature of the damage.
The initial restrictions prevent traffic from going past Kannana-dori (Tokyo Loop Road #7) toward the center of the metropolis and shut down expressways to all traffic except emergency vehicles. The secondary restrictions expand the network of emergency vehicle-only roads.
It’s a good idea to take a look at this flyer (English version)PDF and familiarize yourself with these emergency restrictions, especially if you own a car in Tokyo.