Earlier quoted context omitted.
It's a substantially flatter, straighter line, and much higher capacity. The valley route is only a single lane in each direction with no grade separation at intersections and you are comparing that to a four lane freeway.
> people report saving anything from 20 min to 1 hour by taking the bridge. But during some particular holidays , where there is lots of traffic, the saving can become 4 hours You thing during particular holidays the single lane somehow has even less lanes, less grade separation and such? That would be quite a phenomenon.
In particular most intersections on the now D809 are roundabouts, continuing on the D809 often requires making a turn on the roundabout, and roundabouts are notorious for gridlocking with high turn volumes. Let that gridlock cascade across multiple intersections and you now have rapidly deteriorating travel times.
At other times, traffic is less high so this gridlocking is less likely to occur.