@unpublished{Christiansen2009,
author = {Vidar Christiansen and Stephen Smith},
title = {Externality-correcting taxes and regulation},
year = {2009},
month = {Sep},
doi = {10.1920/wp.ifs.2009.0916},
issn = {1742-0415},
url = {http://www.ifs.org.uk/wps/wp0916.pdf},
}