Whatever that might be, it is on the (R) side of the traffic signal. The car was parked on the (L) side of the traffic signal. And where is the remaining 17 Feet of the car? You can see between the people standing on the (L) of the traffic signal, and the surface street directly behind them is clearly visible. There is absolutely nothing directly behind these people. Not yet.
I know after 62+ years some of you find this hard to believe. But this is what happens when "researchers" are focused on finding shooters, gun barrels, smoke, and any other $$$ generating pulp fiction. And this being the case, boring details such as this car rolling down the Elm St Extension while JFK was under fire are missed.
The car was abandoned in that "NO PARKING At Any Time" zone for 3+ Hrs. What happened to the occupant(s) of that car?
............................................... STAY TUNED ............................................................