That's a good idea, thanks! I was lying in bed thinking about this last night, as you do. MGRS coordinates might work well here: https://en.wikipedia.org/wiki/Military_grid_reference_system for higher zoom levels, then heatmap colour the grid 'squares' with an aggregate sum of accidents, and only add lat/long markers when I get to closer zoom.

At coarse grain, the the coloring could probably be cached and only rerendered on a 'wall calendar' timescale.