Abstract This paper presents results from a study concerning effects and consequences of a temporary (one year) capacity reduction on an urban main road tunnel in Oslo, Norway. The main findings are that reduction of the road capacity from in total four to two lanes in the Smestad tunnel, carrying about 50 000 vehicles per workday, caused few or no effects or consequences. Three months after the capacity reduction was implemented, traffic volumes were about the same as before, there are only small increases in delays, and the road users (commuters and freight traffic) have not made significant adaptions.