An energy storage system having a limited capacity is employed for the purposes of (a) removing the burden of acceleration from the vehicle prime mover in typical urban driving situation and (b) providing an efficient regenerative braking system to significantly increase fuel economy over stop-start driving schedules. Energy is stored in a hydraulic accumulator which is designed to be an integral part of a hydraulic transmission. Data from laboratory tests are presented to indicate achievable energy conversion efficiencies. Computer simulation of various size vehicles being driven over typical urban-suburban driving schedules is used to estimate the reduction in vehicular emissions and energy consumption resulting from the energy storage capability.