Despite the potential for increased fuel efficiency, ICE vehicles alone do not represent a viable long-term solution for decarbonising the urban passenger transport sector for two reasons. First, a significant gap remains between fuel efficiency standards and the real-world operational fuel efficiency of ICE vehicles, which ultimately limits the effectiveness of these types of standards as a measure for reducing emissions. Although fuel efficiency standards have become 30% more stringent between 2000 and 2016, actual fuel efficiency as measured by real world performance, has only improved by 11% over the same period.