One problem related to blind spot detection has been reported for the 2009 Volvo S80. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2009 Volvo S80 based on all problems reported for the 2009 S80.
At night, when a direct strong light (headlight) from behind shines on the car, the blis system stop working. The only way to reset the system is to stop and restart the car, instead of turning the system button off and then back on. This is unsafe since when you are on the highway, it is unsafe to do so. A simple fix is a software update that will reset the system when you turn it off and then on again.