The emergence of Transportation Network Companies (TNCs), e.g. Uber and Lyft, over the last decade has introduced a new form of on-demand mobility that has the potential to be a strong partner with transit agencies, providing lower-cost alternatives where traditional fixed-route transit is not sustainable. As an example, TNCs and transit agencies could partner to create designated zones where the agency agrees to subsidize a portion of all trips taken on TNCs within the zone, rather than providing traditional fixed-route service in that area. By partnering with TNCs, public transit agencies can switch from supply-driven to demand-driven mobility, only paying for drivers when there is demand for a trip. This has the potential to reduce transit agency operating costs, allowing agencies to reinvest resources into higher-performing areas of the network. This model may also allow agencies to merge fixed-route and complementary ADA/paratransit services under one on-demand service model. This research analyzes the cost-savings potential for transit/TNC partnerships in five zones across the Riverside Transit Agency (RTA) service area in Western Riverside County: East Perris, Moreno Valley, Temescal Valley, Calimesa, and Temecula. Each of these zones has either low-performing fixed-route transit service or no fixed-route service at all. However, implementing a transit/TNC partnership is not as simple as switching out operators. RTA must consider factors such as geography, trip length, potential ridership, equitable fares and fare collection, vehicle/driver availability, and federal regulations (Americans with Disabilities Act and Title VI). This research establishes a methodology for evaluating the cost-effectiveness of replacing traditional fixed-route transit services with partnerships with Transportation Network Companies (TNCs), e.g. Uber and Lyft. It evaluates five zones in Riverside Transit Agencyís (RTA) service area that have either very low-performing fixed-route service or no fixed-route service at all. For each zone, the research estimates the average TNC fare for trips taken within a specified zone and compares this estimate to the current subsidy paid by RTA to provide fixed-route and Dial-a-Ride service in the same area. It leverages key lessons learned from five pilot partnerships around the country and provides recommendations for how to structure these partnerships to comply with existing federal regulations and generate ridership.