Urban traffic systems vulnerable to hacking

Washington

PICTURE this: A hacker walks the streets of Capitol Hill with a laptop and malicious intentions. He crouches to street level, tapping into the city's traffic sensors during rush hour. Suddenly, downtown traffic signals turn to a constellation of red. Within minutes, streets are a parking lot jammed with cars. Emergency vehicles, paralysed by traffic, can't find a clearing. Their sirens echo through a gridlocked city.

It's a dark scenario out of a dystopian film, but is it plausible?