Germany and Denmark have agreed to build a bridge linking them, to cut travel time between Scandinavia and Hamburg by up to three hours.
The bridge across the Baltic sea is due to be completed within 10 to 12 years.
Most people travelling from Copenhagen to Hamburg currently use a ferry between the Danish island of Lolland and the German island of Fehmarn.
German ferry ports such as Rostock have lobbied against the new project, saying it will take away jobs from them.
The 19km (12-mile) road and rail bridge is expected to cost about 5.5bn euros (£3.7; $7.4bn).
German Transport Minister Wolfgang Tiefensee said it would "not only significantly cut journey times between Copenhagen and Hamburg, but also is of high value for transport routes in Europe as a whole".