Collecting Traffic Data from Probe Vehicles

PROMOT (PRObe vehicle concept for MOnitoring road Traffic) studies the efficiency of the probe vehicle concept in general and the addressed protocols for uplink probe vehicle transmission, viz. ALOHA and Polling.

Road and Public Transport Network

The starting point of the analysis is a network model of the transportation infrastructure. This may directly be an estimate of average road traffic intensities, or, if this is not available, traffic densities can be estimated using Origin-Destination Matrixes, a road map and a algorithm for estimating which routes drivers will select (traffic assignment).

PROMOT Screen Dump:
Estimated morning-rush road traffic intensity in San Francisco Bay Area

Generation of Traffic Data Messages

A certain percentage of the vehicles serves as probes that transmit traffic messages. The number of probe vehicles on a road link can be assumed to proportional to the computed traffic densities on each road link. We assume that vehicles perfectly know their location, for instance through a hybrid GPS and dead- reckoning positioning technique.

PROMOT Screen Dump:
Intensity of messages transmitted by probe vehicles in San Francisco Bay Area.
Penetration of vehicle with probe facility 1%
One transmission per probe vehicle every 60 sec.

Spatial Throughput

The traffic messages can be received by base stations located in the area under study. In the supply communication network, harmful interference between messages transmitted in the same time slot (collisions) may occur. Message collisions and interference from other cells should be taken into account, e.g. using models for the access method, receiver capture and mobile radio wave propagation. Possible access methods are ALOHA and polling.

Polling:
The process of generating traffic messages under the polling scheme is "demand oriented": The base station transmits a polling message requesting one specific probe vehicle in its cell to responds by transmitting its traffic message. When the probe report is successfully received the base station polls the probe vehicle next in turn. Whenever the transmitted probe report is not successfully received, the base stations re-polls the same probe vehicle and may repeat this process a limited number of times before the base station proceeds with the next probe vehicle.
Polling specific road segments
ALOHA:
Under this supply-oriented access scheme, the process of transmitting probe vehicles messages is even simpler. The probe vehicles are assumed to randomly generate traffic messages, without polling messages from the base station and actually without coordination between transmission from other probe vehicles. As a result, mutual interference between messages transmitted from different probes can occur. Retransmission of messages lost in fading or collisions is not necessary. Because of path loss, messages transmitted in the vicinity of a base station are more likely to capture its receiver than other messages.

Example: Performance of ALOHA

PROMOT Screen Dump:
Throughput of common ALOHA channel with 4 base stations.
Base station location indicated with white circles
Probe vehicle penetration 1%,
One transmission per probe vehicle every 60 sec.
18 time slots per second available.

You may want to compare the cell coverage predicted by PROMOT with a real-world cellular coverage map of the Pacific Bell PCS 1900 system. It shows that a base station located in San Francisco downtown has approximately the same coverage as illustrated by the circle in the above map.

JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards Section: Intelligent Transportation System, Probe Vehicles

Collecting Traffic Data from Probe Vehicles

Road and Public Transport Network

Generation of Traffic Data Messages

Spatial Throughput

Example: Performance of ALOHA

JPL's Wireless Communication Reference Website © 1993, 1995.

Chapter: Network Concepts and Standards
Section: Intelligent Transportation System, Probe Vehicles