In addition, this study does not control for locally imposed TELs due to the lack of data. Locally imposed TELs often constrain local ability in raising property taxes and sales taxes and thus might force local governments to rely more on alternative funding mechanisms such as user charges. This means that the estimated effects on property taxes, sales taxes, and user charges reported in this study could be biased upward. Based on a survey of 320 US cities, Brooks and Philips (2009) suggest that at least one in eight cities, especially large cities with populations over 100,000, has enacted a locally imposed TEL. Further research should control for the effect of locally imposed TELs when data become available.