Much of the jam consists of around 3,000 motorcyclists, reflecting a craze which hit the region a year ago and now draws people from all over the country.
The riders line up from early morning and cross the border carrying about 30 liters of fuel each. Serbian traders then buy fuel on the other side in a slick operation providing both sides with a quick profit.
Fuel carried in vehicle tanks is exempt from the trade embargo imposed against Belgrade for its role in the Bosnian conflict and the riders manage to smuggle through extra fuel hidden in parts of their bikes and their bulky leather jackets.
The number of motorcycles making the trip has risen from 200 a day last year to a current 3,000, a customs official on duty at the checkpoint near the Danube River said.
Up to 7,000 people cross daily on foot or in buses, selling small quantities of consumer goods not covered by sanctions.
The official, who declined to be named, said fears that the embargo might either be lifted or tightened to force a peace settlement in Bosnia had encouraged the traders to make the most of the business while it lasted.
Although the amounts of fuel smuggled are small they added up to around 80-100 tons a day, supporting an area where two thirds of the working population have no jobs.
Bulgaria supports the sanctions, but has bemoaned the loss of trade.
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