Many motorways are running above their capacity and therefore suffer congested traffic. The traffic breakdown from free flow becomes increasingly likely around a critical flow, a critical number of vehicles per minute. Here we discuss congestion durations which are distributed with a power law over two decades, from minutes to hours  (see Figure 1 on the left). This finding suggests a robust mechanism behind it. Using antipersistent stochastic modeling of the traffic flow, we are able to explain the distribution of congestion durations: The traffic flow shows large fluctuations on short time scales which quickly trend back to the mean value (see Figure 1 on the right). Consequently, the traffic flow exceeds the critical flow for time spans which are power law distributed.