While I can see that capacity problems at the Strand/Lancaster Place
junction might be an argument for the underpass to be northbound-only during
the morning rush-hour, and possibly the evening rush hour too, it seems
obvious to me that it should be southbound-only at all other times. There's
no real need for a northbound underpass when the roads are not crammed full,
and a southbound underpass would cut about 300 metres off a lot of journeys,
saving a lot of time, money and fuel. Tidal flow might have been rocket
science 50 years ago, but it surely couldn't cost much these days.