A new hybrid vehicle prototype could make cargo transportation a lot more energy efficient. The add-on system uses a special trailer that provides a power assist on hills and uses regenerative braking technology to recharge the onboard battery system. The system works similar to a hybrid car; the motor generators produce electricity via regenerative braking instead of normal friction brakes, and a bank of batteries can be mounted under the trailer chassis to store this energy. When the truck/trailer combo needs to accelerate or go uphill, electric power flows from the batteries to the motors to provide some or all the power needed to propel the truck. It is one of several potential systems that aim to increase fuel economy and reduce the carbon emissions from heavily polluting tractor trailers.