The numbers have been crunched (hat tip to NPS) and there was quite a bit of evening activity on July 4th at the Crystal City Connector trail that runs from the Mount Vernon Trail by the airport, to Crystal Drive by the water park. This connector trail is the easiest way to get from Crystal City out to Gravelly Point for picnicking and firework viewing. See the counter location highlighted in yellow below:

Now onto the numbers! In the below graph (click to embiggen) you’ll see the first seven days of July, all of which have pretty similar looking peaks in the 100ish trips range. But then there is the 4th, which around 10:00 pm peaked at more than 800 sensible people using bicycles or their feet instead of cars to view the fireworks.

The highlighted numbers displayed are for the 10pm – 11pm hour time frame.