An employee was valeting and checked in a customer's vehicle that had been modified to accommodate the customer's physical disability. The employee went to move the vehicle from drive when it accelerated unexpectedly. Due to the modifications on the vehicle, the employee was unable to locate the brake pedal. In an attempt to avoid two vehicles and an ambulance parked on the drive, the employee swerved but was unable to avoid the vehicles. The employee struck several vehicles before hitting a wooden/aluminum shelter and a concrete wall and finally coming to a stop. The employee sustained a knee injury and was hospitalized for surgery.