The Ontario Ministry of Transportation has installed 27 changeable message signs (CMSs) strategically upstream of express–collector transfer locations on Highway 401 in Toronto, Ontario, Canada. Motorists are informed by the CMSs of traffic conditions downstream of the transfer location to help them decide whether to take the next transfer. Loop detectors are installed at the transfer locations to measure traffic flow. The dynamic impacts of CMS messages on traffic diversion are evaluated by using 3 years of loop detector data, from 2003 to 2005. Time-series plots of aggregated diversion rates show that the initial transient response to the message change is significant and that, in addition to the messages themselves, the occurrence of a message change plays a vital role in influencing downstream diversion. Aggregate diversion rate plots also reveal that the logic underlying the Highway 401 CMSs may be causing the signs to react to diversion rate changes on many occasions, thereby closing a feedback control loop that effectively regulates the downstream diversion rates.