Those conditions are not particularly bad compared to other road bridges constructed in the 1990s and since.
For example one of the bridges in Hong Kong has a fully enclosed pair of traffic lanes to be used during the build up to a Typhoon.
But the hybrid bridge-tunnel system was only one of the proposals, there was an entirely bridge one as well.
All this does show, however, that it wasn't the case that the governments just decided to build a rail tunnel.
Various schemes which would have permitted vehicles to drive across were considered and rejected (rightly or wrongly).
If I recall correctly, there was even a provision that a certain number of years after the tunnel was built the operator had to consider the feasibility of building a parallel road crossing. It was considered unfeasable but of course that's in the context of already having a rail tunnel.
I find it hard to see though how any of these schemes could have come close to the extremely high safety standards required of the channel tunnel - but maybe as road schemes they wouldn't have been expected to.