High occupancy vehicle lane access can be an important non-monetary incentive for increasing advanced clean vehicle sales. This incentive needs to be balanced against the potential cost of increased congestion on those lanes, especially during peak travel periods. In California, there are two types of HOV access: 1) White decals, which are available to an unlimited number of qualifying federal inherently-low-emission vehicles, mostly 100% battery electric and fuel cell electric vehicles (BEVs and FCVs) and 2) green decals, which are available to the first 40,000 applicants that purchase or lease cars meeting California's transitional zero emission vehicle (TZEV) requirement, mostly plug-in hybrid electric vehicles (PHEVs). The expiration date for both the green and white decals is 2019. The green decal quota is expected to run out before June 2014 with more than 36,000 issued between 2011 and March 2014. Current discussion focuses on whether to add more green decals beyond the current limit of 40,000. More decals will sell more PHEVs but will also increase the number of cars on HOV lanes and may reduce the lane performance and the revenue of high occupancy toll lanes (HOT). By developing a better understanding of the costs and benefits of HOV decals as an incentive, we can better understand how to tailor policy for maximum benefit. Our analysis shows that PHEVs with greater all-electric range provide much higher levels of zero-emission travel per HOV mile. Policy makers can take this factor into account when determining the level and duration of the HOV incentive.