There is too much uncertainty to give a date for when a Clean Air Zone be implemented, should that be the outcome of the technical work being undertaken 2018-19. The timescale will be dependent on the scheme selected and the many factors influencing difficulty of implementation. We believe that the soonest any of the UK cities can achieve implementation of a CAZ will be 2021.  Bristol started after the first wave of cities so it is likely to be after that. Bristol will aim to achieve compliance in the shortest time possible.