In a recent publication by the Chair of Transport Ecology on behalf of the Scientific Climate Council of Hesse, the effects of the expansion and new construction of highways were examined. The researchers came to the conclusion that the expansion of trunk roads leads to significantly more traffic than assumed in the current Federal Transport Infrastructure Plan. On the other hand, it leads to an increase, not a reduction, in CO2 emissions. Possible savings due to a liquefaction of traffic are overcompensated by increased traffic and higher speeds.

The researchers conclude that the traffic induced by road expansion is systematically underestimated in the Federal Transport Infrastructure Plan (BVWP). As a result, significantly higher CO₂ emissions than previously assumed are to be expected when implementing all road projects planned in the BVWP. Depending on the scenario, these emissions can be up to three times higher than forecast when the plan was drawn up.

A specific problem becomes apparent in conurbations such as the Frankfurt/Rhine-Main metropolitan region. Road expansion projects planned there induce particularly high levels of traffic. Implementation of the projects can contribute to traffic jams shifting into the conurbations and placing additional strain on inner-city road networks. This would counteract local efforts to reduce road traffic, shift traffic in favor of eco-mobility and make cities more liveable.

Against this backdrop, the study recommends making greater use of nationwide measures to influence road traffic demand in order to reduce or avoid congestion on the trunk road network. These include measures to expand public transport services, traffic pricing and speed limits. In conurbations in particular, alternatives to road expansion should also be systematically investigated and supported in an integrated manner. As part of integrated transport planning, measures to make local public transport, cycling and mobility management more attractive should be included.

The results of the study suggest that planning and funding within the framework of the Federal Transport Plan should be further developed into integrated federal transport infrastructure and mobility planning(see study on this topic). This should contribute to consistently aligning the transport system with climate targets and comprehensively taking all necessary measures into account.