The Shopping Line Effect is this: When you're at a grocery store, and there's one lane open, everyone who needs to check out is obviously in it. You open another and customers from the original line will immediately jump out and line up there until the two lines are essentially equal. The more check-out lanes you open, the shorter the lines get, but -- and this is the important part -- the customers will almost ALWAYS self-equalize the lines as a means to get to the counter and, ultimately, the exit.
Grocery store lines, like life and dinosaurs, find a way.
Simple enough, huh? Obvious too...we all do it. I can't quantitatively tell you its connection to Economics (something about supply and demand?), but I think it certainly applies to the Interstate-70 issue and the effect removal of its Poplar to Cass would have.
There has been a lot of conversation about the traffic impact of City to River's boulevard plan -- feasibility studies, back-up/congestion concerns, issues of flow and bottle-necking. Actually, just today Gateway Streets approached the subject (Link complements of STL Rising). Taking the Shopping Line Effect into account, it stands to reason that, even if there was back-up on a new Memorial Drive boulevard, the drivers moving north and south, from 70, 55, 40, 64, 44 and all the numbers of the rainbow would utilize the surface streets of downtown St. Louis, north St. Louis and Midtown.
This, in almost entire likelihood, will require a few more exit ramps and - gasp! - the adjusting of speed limits along these Interstates as you approach the ramp options, but people will adjust. And who knows, as more people move up and down the roads -- rather than above and past them -- these once "unsavory" or "blighted" or "undevelopable" lands and areas will be rediscovered and re-emerge as an extension of downtown or thriving neighborhoods in their own rights.
Shameless plug for City to River (like you all don't know about it already), but they really have a great plan ready for the city to adopt -- aesthetically, functionally and fiscally (and other "ly's" too). We're two months away from getting to see whether the City and the designers it has given this responsibility to think the same.