Top: Average number of photos/tweets per 100-foot-square area for areas with each indicated vehicle speed.
Bottom: Scatter plot of all photo/tweet counts vs. all vehicle through speeds for 100-foot-square areas, with 1 unit of noise in the photo/tweet count to reduce banding.
9 miles per hour appears to be the sweet spot of optimal traffic speed for popularity, although there are many locations that no one photographs or tweets about at every speed. (This is actually a very common speed for streets after you take the time spent stopped at intersections into account. Peak speeds on these streets are undoubtably higher.)
The outliers at 40-50 mph are locations on the Golden Gate Bridge, showing that an iconic structure and scenic views can overcome the negatives of high traffic speeds.
It's also hard to separate cause from effect -- is the traffic speed moderate at popular places because there are lots of people around, or are there lots of people because fast traffic isn't chasing them away? Probably some of both.
Speed data from Cabspotting. Popularity data from Flickr and Twitter.