Unless they're New York black holes, in which case they look to make sure there's at least .5 nanometers of extra room between cars in the other lane then quickly and erratically swerve over before the trailing car can agressively close up the gap (which he will automatically try to do once he sees the manuver start). If successful the black hole will then immediately slow down to 10mph under the speed limit and glory in the angry honking coming from behind, if unsuccessful the black hole will give the blocking car the finger which they won't see because... IT'S A BLACK HOLE!
I would tend to think that two black holes merging will work roughly the same as any large mass being swallowed, the resulting black hole will be the combined mass, the event horizon will shift appropriately, the spin and vector will be based on the two inputs... and you won't see ANY of the action.
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