I thought the whole roads make cars thing was a case of correlation being confused with causation? The UK provides a case study that tend to discredit this theory.  Our government bought into the concept some 30 years ago and major road construction projects were slowed right down.  30 years on we&#x27;ve had little slowdown in growth of car numbers and now have even more congested roads.<p>What is true is that traffic flow does not depend on e.g. number of lanes.  The problem of traffic flow has to be addressed on a system level rather that a individual street level.  So e.g. variable speed limits can overall keep traffic flowing more smoothly than adding a lane to a freeway&#x2F;motorway would.