I was just now doing some research on bus ticket clearing houses and I came across his post in rec.travel.europe from 2001 by Michael Forrest.

 >Reciprocity is not guaranteed on airlines (or toll bridges - it
 >has always amused me that the toll on the Severn Bridge between
 >England and Wales is GBP 4.20 for a car to enter Wales but there
 >is no toll the other way, ie entering England). (Maybe it shows
 >how the two countries value themselves?)

All of the bridges in the San Francisco Bay area only charge toll in one direction. Some genius realized that, on the average, just about as many cars went each way for the obvious reason that most trips across the bridges are round trips. So they doubled the toll and took down the toll booths going one way (except for the Golden Gate Bridge where the toll booths remain, unattended). There has been considerable saving in toll collection costs and the toll booth traffic jams in one direction are gone. Is it possible the Severn Bridge does similarly?