  The EPC GEN 2 communication protocol for Ultra-high frequency Radio Frequency
Identification (RFID) has offered a promising avenue for advancing the
intelligence of transportation infrastructure. With the capability of linking
vehicles to RFID readers to crowdsource information from RFID tags on road
infrastructures, the RF-enhanced road infrastructure (REI) can potentially
transform data acquisition for urban transportation. Despite its potential, the
broader adoption of RFID technologies in building intelligent roads has been
limited by a deficiency in understanding how the GEN 2 protocol impacts system
performance under different transportation settings. This paper fills this
knowledge gap by presenting the system architecture and detailing the design
challenges associated with REI. Comprehensive real-world experiments are
conducted to assess REI's effectiveness across various urban contexts. The
results yield crucial insights into the optimal design of on-vehicle RFID
readers and on-road RFID tags, considering the constraints imposed by vehicle
dynamics, road geometries, and tag placements. With the optimized designs of
encoding schemes for reader-tag communication and on-vehicle antennas, REI is
able to fulfill the requirements of traffic sign inventory management and
environmental monitoring while falling short of catering to the demand for
high-speed navigation. In particular, the Miller 2 encoding scheme strikes the
best balance between reading performance (e.g., throughput) and noise tolerance
for the multipath effect. Additionally, we show that the on-vehicle antenna
should be oriented to maximize the available time for reading on-road tags,
although it may reduce the received power by the tags in the forward link.
