It happened around 4:41 p.m. Saturday on Route 1 southbound.
Police say a 76-year-old Lewes man was operating a 2005 Ford Econoline van, pulling the Jolly Trolley, carrying about 22 passengers.
For an unknown reason, the trailer started to sway and then overturned.
The van stopped and bystanders in the area assisted with pulling passengers from the trolley.
At least 12 people were taken to area hospitals, where they were treated for nonlife-threatening injuries.
The driver of the van was uninjured.
Route 1 southbound was closed for about two hours while the crash was investigated and the scene was cleared.
No citations were issued.
The crash remains under investigation. null