This paper investigates to what extent travellers change their route when faced with unexpected traffic situation. To this end, traffic data from days with serious incidents are analysed in this contribution. The flows retrieved from loop detectors on the routes past the incident and on alternative routes are compared with the same values on days without an incident. It is found that for major accidents up to 50% of the travellers deviate from their normal route if the traffic situation is different. Furthermore, more travellers take an alternative route if the delay on the original route is caused by an accident than if they are faced with the same delay on the original route without an incident. These findings are for instance important for providing route information or suggestions on alternative routes or for finding vulnerable links.