I'm sure he was going for some "midnight delight" on the beach with the gal.
...Delamore makes the case that he was fleeing from a summer constable, who, though in a police uniform, had no arrest powers and would likely only have tut-tutted about a married middle-aged Senator out with a young woman late a night. Kennedy had offered to drive Mary-Joe to the Edgartown ferry (it's about a 100-yard wide channel), apparently stopped and was approached by a uniformed summer constable, who thought he was lost tourist. (Chappaquick is featureless and the roads are poorly marked.) Kennedy panicked and took off at 60 MPH, and vaulted off the narrow Dyke Bridge, which at the time had no guard rails, only a pair of 2 x 10's nailed flat on top of one another. (The new Dyke Bridge has a 3' high guard rail constructed of 8 x 8 timbers suppored by a 1" steel cable running the length of the bridge and anchored at both ends.)
Kennedy would panic at an officer - he was in a hurry to get to the beach - not the ferry. any idiot who was ever there knows the difference between the main road and a very bumpy/gravel Dike Road..
Sounds like the city council should hold a vote to name the new bridge after Mary Jo Kopechne (e.g. "The Mary Jo Kopechne Memorial Bridge").