The Pitt raiders placed mines and other traps to stop Pitt slaves from escaping across the bridge. A lone sniper effectively eliminates any the mines miss.
If the wrecked vehicles scattered about on the bridge are an indication, it seems that shortly after the war National Guard troops attempted to enter Pittsburgh via the bridge but were stalled by heavy traffic as survivors attempted to flee the city.
At the south end of the bridge, there is a wildmen camp. Also, not far from this end there is a Pitt raider with a sniper rifle on a jury-rigged walkway. The northern end of the bridge serves as the entrance to The Pitt Downtown.
The shoreline and water are more irradiated than any body of water in the Capital Wasteland (around 350 on the river bank and around 600 in the water). Drinking the water yields another 350 rads, but no health. In general, the farther the Lone Wanderer go in the river, the more rads he/she gains, even up to 2100 rads/sec.
pc If you do explore the heavily irradiated area around the bridge using console commands, such as tgm, the bridge above will become heavily and permanently irradiated.
There is a raider sniper at the very top of the bridge. If the Lone Wanderer kills the sniper and climbs up the scaffolding on the pedestrian walkway, he/she can get the sniper's rifle and loot several boxes of ammunition. Even if the Lone Wanderer side with Ashur, this Pitt raider is always hostile.
The snipers nest has a bed that offers the Well rested perk effect.
Even if the Lone Wanderer has disarmed the mines that kill the runaway slaves, they will still be killed by hidden, scripted explosions.
The Pitt Bridge appears to be modeled after the South 10th Street Bridge of Pittsburgh, which in itself seems to take styling cues from San Francisco's Golden Gate bridge, but was in fact built earlier.
According to The Pitt Downtown terminal entries, this bridge is a reconstruction of the Wabash bridge, "replacing the historical Wabash Bridge with a simple copy of the 10th Street structure."
The Pitt Bridge may be a reference to the bridge at the end of the movie Escape from New York which is also heavily mined and covered in abandoned vehicles.