A mountain pass can be open at breakfast and gated by mid-morning with no storm visible from the valley floor. The decision is made on conditions above the road, not on the road itself.
The road surface is not the hazard
Most pass closures are ordered because of what sits on the slopes overhead. Snow that has accumulated on a steep face is the thing being managed, and the tarmac is simply where it would land.
Highway crews therefore watch the snowpack rather than the surface. A pass can be dry and perfectly driveable at the moment it is closed, which is why the order looks arbitrary to drivers.
The same logic applies to rockfall in warmer months. Freeze and thaw cycles loosen material well above the carriageway, and the closure follows the loosening rather than any visible debris.
Snow loading changes faster than weather
Wind moves snow that has already fallen, which means a slope can load heavily on a clear day. A ridge acts as a conveyor, stripping one aspect and depositing the load on another.
That deposition is what shifts a slope from stable to marginal. The change can happen over a few hours, far quicker than a public forecast updates.
Drivers experience this as a closure that contradicts the sky above them. The sky is honest, but it is not the variable that governs the decision.
Control work has to be scheduled
Where avalanche paths cross a road, crews bring slopes down deliberately rather than waiting for them to fail. The work is done with charges or from a helicopter, and the road must be empty while it happens.
Scheduling that window depends on visibility, wind and staffing, all of which move. A control shoot planned for dawn slips to late morning, and the closure slips with it.
Once material is on the road, clearing takes longer than the shoot did. A closure announced as brief extends because the volume that came down was larger than expected.
Gates sit where turning around is possible
Closure points are chosen for geometry rather than for proximity to the hazard. A gate needs somewhere a truck can turn, somewhere vehicles can queue, and ideally a settlement with fuel and food.
That places many gates a long way below the actual problem. Drivers stopped in mild weather at low elevation are being held back from something they cannot see.
It also explains why detours are enormous. The next crossing may be a long day away, because the terrain that forces one pass also forces the spacing of all of them.
Reopening is slower than closing
A pass shuts the moment a threshold is crossed, which takes minutes. Reopening requires inspection of every path along the route, and inspection cannot be done in poor visibility.
Crews also have to be satisfied that the slope has finished moving. A second release onto a road full of traffic is the outcome the whole system exists to prevent.
So the reopening time given at a gate is an estimate built on a forecast. Treating it as a schedule is what leaves drivers stranded in a summit town overnight.