While driving 7th East along Liberty Park, I found the traffic pattern strange. I'd turned right onto the street, during a green light, and before me lay a vast expanse of unused pavement, and cars bunched together, waiting for the light at 13th South to turn green.
It made me realize that most roadway congestion occurs at intersections, where everyone has to stop and wait. The larger the intersection, the worse it gets, because there are more people waiting who have to get going. And all the while, the area between the intersections is bare of cars. Anyone who has ever driven the Bangerter Expressway is aware of the phenomenon.
That suggests that more efficient use of roads could be made if some cars were allowed to cross the intersection while the other street of the intersection was turning. This could be made possible through the use of a single-lane single lane tunnel.
A tunnel is typically an expensive beast, largely because it is engineered to handle very heavy, very large vehicles--18 wheelers and the like. This requires them to be engineered to much higher weight and slope standards then the normal passenger car requires. If a single-lane tunnel were engineered for passenger cars only, it could be much shallower, steeper, and shorter--and thus much cheaper.
Only one lane in each direction would need to be tunneled--the other 2 lanes in each direction would function normally, subject to normal intersection delay. But it would do a great deal to facilitate 'queue-jumping'.
It has been done elsewhere, as a solution for very large intersections. Really, once you hit Primary Arterial status (Redwood Road, State Street, 7th East) where the two way left hand turn lane has been removed and replace with a barrier, you might as well sink a couple of lanes. Sinking the lanes would also reduce the last-minute pre-intersection lane changes that are the cause of so many accidents.