Inflation-adjusted federal funding of transportation projects per vehicle mile of travel has been decreasing for decades. In response, states and regions around the United States have scrambled to make up the shortfalls. In the nation's most populous state, California, the 18-cent per gallon motor fuel excise tax remained unchanged between 1993 and 2016, despite the fact that inflation and increased vehicle fuel efficiency together eroded the fuel tax's buying power (1). Since the 1980s, the growing gap between transportation program needs and revenues has been increasingly backfilled in California (and several other states) by county sales tax measures for transportation. Since 1976, California residents have voted on 76 local option sales taxes (LOSTs) to fund transportation in 30 of the most populous of the state's 58 counties. As of 2017, 24 counties, home to 88% of the state's population (2), have active LOST measures. The most recent measures in these so-called Self Help Counties have been approved by at least two-thirds voter majorities as required by California law. Transportation sales tax measures can produce substantial revenue for the maintenance, operation, and expansion of transportation facilities and services; sales tax revenues dedicated to transportation today produce over US$4 billion per year for transportation construction and maintenance in California (3). Voters in some counties have approved LOST measures as many as five separate times (4). LOSTs for transportation have proven politically popular; voters tend to like them because they fund popular projects via taxes automatically levied in very small increments (often a ½-or 1-cent per dollar) over a very large number of transactions. Because these small levies apply to nearly all consumer purchases, LOSTs generate substantial revenue for transportation. On the other hand, LOSTs are less related to travel than are fuel taxes or tolls, which means that light users of transportation systems tend to pay more per mile in transportation sales than do heavy users (5). Despite LOST's increasing role in transportation finance, there is no comprehensive, up-to-date data source on transportation sales tax measures. Given the labor intensiveness of gathering comparable data on LOST measures, and in particular on what they fund, this research focuses on the most 782757T RRXXX10.