The COVID-19 pandemic has spread globally; however, the risk of contracting COVID-19 on public transportation and its role in local spread remains unclear. Essential workers who are transit-dependent tend to be from low-income and minority populations and are faced with the risk of contracting COVID-19 each time they take a bus. We investigate bus ridership from April to September of 2020 and the risk of contracting COVID-19 on the bus by combining a transportation data analysis and an epidemiological model of COVID-19 risk. Our results show that 4% of county cases were contracted on the bus or from a bus-rider (first degree), disproportionately (52%) from trips that were over their mandated capacity. The risk of contracting COVID-19 on the bus was low, but socially worth mitigating. A cost-benefit analysis reveals that dispatching autonomous vehicles or deploying longer buses for passed-by passengers rather than allowing crowding have the lowest societal costs.