Of the injured, 18 were in serious condition.
A presidential election to choose a successor was delayed three times before finally taking place on December 30, the same day as the legislative poll.
The December 30 vote saw 21 candidates run to replace President Joseph Kabila, who has ruled the vast, conflict-ridden country for almost 18 years.
Two and a half times bigger than France and Germany combined, the Democratic Republic of Congo has a long history of political turmoil and violence.
The accident occurred on a highway linking the capital Kinshasa to the country's sole seaport at Matadi on the Atlantic Ocean.
In May, 50 people died in a similar accident on another river in the northwest of the sprawling former Belgian colony.