The 1980 Mt. Saint Helens eruption is an intersting case study in the problems officials have in calling for an evacuation. The problem wasn't so much that the warning was too late. Rather the problem was that people didn't believe it.

In 1980, there were strong indications that a major eruption of St Helens was imminent, and the situation was very dangerous. The USGS convinced the state, counties, and USFS to close the area around the mountain. The problem was that a lot of people had cabins in the area, and there was a lodge and other businesses in the area. When after some time there was no eruption, the public pressure to re-open the area became intense. People were calling the USGS all sorts of nasty things, and demanding that the closure of the area end. Lots of people were ignoring the closure and sneaking into the area.

The authorities finally gave in to that pressure, and opened the area for one weekend, to allow people to go to their cabins and retrieve their belongings. That was the weekend the mountain erupted, and 57 people died. The eruption was in fact much worse than even the USGS expected. The blast flattened forests for miles, mud flows destroyed bridges and partially blocked the shipping channel in the Columbia River, and ash clouds covered a huge area downwind.

A very good read about St. Helens is the book "Volcano Cowboys: The Rocky Evolution of a Dangerous Science" by Dick Thompson. He describes in great detail the science behind the warnings, and also the problems the USGS had in getting the public to take the whole thing seriously.

I grew up in the PNW, and climbed Mt. Saint Helens several times in the years before the eruption. By the time of the eruption I was living in Denver. We actually got a tiny but visible bit of dust even there from the eruption. If you are ever in the area, it is well worth it to drive up there and visit the Johnston Ridge Visitors Center (named after a USGS geologist who died in the eruption). It still boggles my mind to look up there and realize that most of the peak I climbed is....gone.