metrognome
ok, first of all, it’s the State, not the city. Second, the signs are part of a comprehensive effort to create a ‘smart highway’ system around the Puget Sound, which has one of the worst traffic overload problems in the country. Other parts of that effort include the overhead lane closure signs to divert traffic around stalls and wrecks, the traffic cameras, info available via smart phones, etc. Third, it appears altho the signs haven’t yet helped reduce congestion, they have helped reduce rear-end collisions:
“A total 35 crashes occurred in the seven-mile stretch between Tukwila and Interstate 90 from Aug. 10 to Nov. 1, compared with a range of 100 to 140 wrecks between Aug. 10 and Nov. 10 the past five years …
“Drivers are constantly asking what’s the point of having a sign that says 40 mph, when I’m stuck in stop-and-go traffic?
….
The first answer, says McCormick, is that 40 mph becomes the legal maximum, and troopers have handed out speeding tickets. (Minneapolis’ speeds are merely advisory.) As with any road, drivers must react safely to conditions. Nonetheless, even an inflated “40 mph” figure still gives a clue congestion is ahead.
Second, the signs can be only so precise. “A lane that was doing 30 a few seconds ago might be going 10. It’s really dynamic,” McCormick said.”
from http://seattletimes.nwsource.com/html/localnews/2013373837_smartsigns08m.html
The story includes more details on the state’s efforts to manage our freeways more efficiently.