Metro ExpressLanes are designed to improve the traffic flow and provide motorists, including solo drivers, a more reliable travel option in LA County. ExpressLanes allow carpools, vanpools, and buses to travel for free, while also providing single occupant vehicles the option to pay a toll to use the lane. Tolls for single occupant vehicles are calculated based on traffic conditions and vary accordingly to the level of congestion – tolls are higher when traffic congestion is heavier and lower when traffic is lighter. Use of ExpressLanes requires a FasTrak Flex transponder mounted on the vehicle's windshield. Currently, Metro operates ExpressLanes on I-110 and I-10 freeways in LA County. Other facilities are being studied or are in the design phase for expansion of the network.