Sorry to be so Seattle-centric..this post about Seattle's Alaskan Way Viaduct got me thinking.
If the Viaduct is closed--whether for construction of a tunnel or a new aerial highway, or to make way for green space and a surface street--what happens to rush hour?
As far as I can tell, it boils down to this: without the Viaduct, transportation planners will have to figure out how to accomodate the equivalent of 11,000 rush-hour car trips through the busiest part of downtown. The data include figures for average weekday traffic, plus the traffic volume during the one-hour morning and afternoon peaks.
(To my surprise, it seems that the tunnel may have been underutilized; one estimate, a few years old now, is that the tunnel could carry 18,000 trips per hour (scroll down a bit to find the claim), with everyone seated, in buses alone.
If that's possible, then the tunnel alone would be meet the post-Viaduct demand.) And then there's Third Ave., which is currently closed to cars during rush hour.
An upcoming report by the Association of Bay Area Governments projects the city of San Francisco to add a record-breaking number of residents by 2040.
The SF Examiner is running a week-long series exploring the impacts of the expected growth.
With no Viaduct, peak-hour travel demand will increase by somewhere between 6,100 and 7,200 trips. The existing street grid may be able to hold a few more cars than it currently does.
(Note, this is a somewhat conservative estimate -- I'm assuming that some former Viaduct trips will "disappear"--i.e., move to other times or destinations--because they'll take too long; but I'm not assuming the same for trips already on the surface streets.) So that's the one-hour peak. Some tweaks to traffic enforcement ("don't block the box"), elimination of some street parking during rush hour, and so forth may increase throughput a bit more.Traffic studies show that the Viaduct carries about 105,000 daily trips.But most of those trips are at off-peak times when the surface streets have plenty of extra capacity.Once the tunnel reopens, it will be able to handle about 9,000 rush hour trips that right now are travelling on Third Ave.And when light rail starts running through the tunnel, its capacity could grow by a third or more.So the thorniest problem that traffic planners will have to face will be accomodating rush hour trips on the street grid and I-5, during the busiest part of the day. And what are the options for dealing with the added load?