Various road user charging mechanisms are used to control traffic and its resulting pollution, as well as revenue sources for reinvestment in the road infrastructure. Among them, electronic toll collection (ETC) systems based on user positions estimated with Global Navigation Satellite Systems (GNSS) are particularly attractive due to their flexibility and reduced roadside infrastructure in comparison to other systems such as tollbooths. Because GNSS positioning may be perturbed by different errors and failures, ETC systems, as liability critical applications, should monitor the integrity of GNSS signals in order to limit the use of faulty positions and the consequent charging errors. The integrity-monitoring systems have been originally designed for civil aviation; hence, they need to be adapted to the ETC requirements. This paper studies the use of receiver autonomous integrity monitoring (RAIM), which are algorithms run within the GNSS receiver and, therefore, are easier to tune to ETC needs than other systems based on external information. The weighted least squares residual RAIM used in civil aviation is analyzed, and an algorithm modification for ETC is proposed. Simulations demonstrate that the proposed RAIM algorithm has a superior level of availability over civil-aviation-based RAIM procedures, particularly in urban environments.