When did the snow start? (from John Toner)

The snow falls onto a long street at a constant rate. At midnight, a snow plow started to remove the snow from one end of the street with a constant volume/time rate. At 1am, a second snow plow started from the same end, removing the snow at the same rate. At 2am, a third snow plow started. At a moment, all three of them met.

When did the snow start to fall?