My friend, who is a biologist and knows these things, says an alligator can run 25-30 mph though it's unlikely it would have chased after a cyclist. More than likely, he would have dived back into the ditch as he did when the cars came by. A caveat however: the month of May is the peak of gator mating season and they tend to act more aggressively during this time because they're looking for mates. So who knows? Good thing the cars came by and good thing you saw it. I'd be on the lookout along that path over the next few weeks, though, thinking you're not looking for a gator mate yourself. ;-D Be safe out there.