Vehicle was set in tow/haul mode. Tow/haul mode provides higher acceleration by changing the operation of transmission. Vehicle was towing 2500lb cargo trailer on interstate. Driver stopped at rest area and turned off engine and left vehicle. Driver returned to vehicle 5 minutes later. Driver started engine and proceeded to leave the rest area. Driver was on highway entrance ramp. Entrance ramp merges with right lanr Driver attempted to accelerate to enter highway in such a way that there would be safe distance between end of trailer and car on highway approaching end of trailer. Vehicle would not accelerate as quickly as it was accelerating for the past 2 hours. Driver was surprised and distracted. Margin of safety between rear end of trailer and car on highway was greatly reduced. Some time after the incident Driver realized that car forgot it was in tow haul mode whenever engine is turned off. There is no warning that mode changed. This means that car does not behave as reasonably expected after an ignition cycle