  Positioning is expected to be a core function in intelligent transportation
systems (ITSs) to support communication and location-based services, such as
autonomous driving, traffic control, etc. With the advent of low-cost
reflective reconfigurable intelligent surfaces (RISs) to be deployed in beyond
5G/6G networks, extra anchors with high angular resolutions can boost signal
quality and makes high-precision positioning with extended coverage possible in
ITS scenarios. However, the passive nature of the RIS requires a signal source
such as a base station (BS), which limits the positioning service in extreme
situations, such as tunnels or dense urban areas, where 5G/6G BSs are not
accessible. In this work, we show that with the assistance of (at least) two
RISs and sidelink communication between two user equipments (UEs), these UEs
can be localized even without any BSs involvement. A two-stage 3D sidelink
positioning algorithm is proposed, benchmarked by the derived Cram\'er-Rao
bounds. The effects of multipath and RIS profile designs on positioning
performance are evaluated, and several scenarios with different RIS and UE
locations are discussed for localizability analysis. Simulation results
demonstrate the promising positioning accuracy of the proposed BS-free sidelink
communication system in challenging ITS scenarios. Additionally, we propose and
evaluate several solutions to eliminate potential blind areas where positioning
performance is poor, such as removing clock offset via round-trip
communication, adding geometrical prior or constraints, as well as introducing
more RISs.
