The A57, a heavily used commuter route in the Rhineland, was completely closed, and will remain shut in both directions between Dormagen and Neuss-Süd until at least the weekend, the regional NGZ news portal said.
A further 16 people were injured, some seriously, and were taken to nearby hospitals. The motorway is covered with wreckage from the vehicles, which included a car transporter full of new cars.
Engineers were called to the bridge to stabilise it, as there were fears it could collapse. The police said the fire - burning plastic pipes lying on asphalt - underneath had created such a heat that large pieces of concrete had fallen from it.
“We are currently investigating who did this and how they did it,” a police spokesman told the NGZ. He said evidence had been collected from the scene which would be used in the investigation.
The A57 is one of the most important commuter routes in the Rhineland area, stretching 128 kilometres from Cologne to the Dutch border. More than 100,000 vehicles travel down parts of the route each day.