One crash on a 2 km four-lane highway, replayed from ns-3.45. Same traffic in every strip; only the rule each car uses to re-broadcast the alert differs.
At 3 s the highlighted car crashes and blocks the eastbound carriageway (traffic keeps left, so eastbound is the upper half). It sends an ACCIDENT ALERT as a UDP broadcast every second. Every other car runs the same socket application, which decides whether to pass the alert on.
Blind flooding re-sends on the first copy, so dozens of cars transmit at the same moment and their frames collide. Gossip re-sends with probability 0.6, and on a thin highway the alert often dies out. Smart Relay waits less the farther it is from the sender, and cancels when it hears a car further along already relay.
Hold Ctrl and scroll (or pinch) over a road to zoom, and drag to pan. Space plays or pauses, and ← → jump between alerts.
Delivery is measured over cars on the 2 km section when each alert is sent. Averages over 10 seeds and 6 densities are in the report.