Viewpoint: Getting on the WAVE of road safety

Viewpoint: Getting on the WAVE of road safety

The traditional mode of highway operation rests on a paradigm of a statically configured right-of-way (the highway with its entrances, exits, lanes, speed limits, etc.), and a collection of autonomous agents (vehicles with their drivers), who must all individually thread their way from origin to destination, each governed by the same regulations and operating constraints intended for maximum safety and convenience for all.

This paradigm is about to change.

The pending deployment and utilization of Intelligent Vehicle Highway Systems (IVHS), which use Wireless Access in Vehicular Environments (WAVE) standards developed by IEEE and SAE, will allow vehicles to begin exchanging critical operational information, thus enabling them to effectively cooperate on the roads in ways never before possible. Not only can we avoid hundreds of billions of dollars annually in property and casualty costs, the increase in highway traffic volumes due to improved cooperation between vehicles can improve travel times and reduce fuel consumption and save money on infrastructure improvements.

WAVE in vehicles soon

WAVE-enabled vehicles are expected to begin appearing in production in the next 2-3 years, though it may take more than 20 years for all vehicles to be suitably instrumented if we depend on newly manufactured vehicles only to turn over the fleet. Ford has already announced that it will equip some of its vehicles very soon with WAVE brake activation. Other vehicle manufacturers are expected to follow.

The problem is that, although successful tests have been conducted by the U.S. Department of Transportation in cooperation with the automakers, the test conditions have not reproduced the worst possible cases of heavy traffic congestion. And it is under such conditions that the reliability of the technology remains in question.

The connectivity between vehicles in proximity to one another, at any given time, is enabled by the availability of a sliver of RF spectrum over which any vehicle can transmit information to one or more of its neighbors, allowing them to maintain a real-time knowledge of their surroundings. The terminology adopted by the industry for this inter-vehicle communications is V2V (vehicle-to-vehicle).

However, only one transmitter can currently use that sliver of spectrum, formally a channel, at any given instant. Otherwise, transmissions would collide, effectively “jamming” one another. As a result, any scheme adopted to allow shared use of an RF channel must either be based on strict timing constraints imposed on all transmitters (deterministic) or pseudo-random scheduling of transmissions (stochastic).

Spectrum constraints

The dynamic mobility of vehicles – relative to one another – would make a deterministic scheme extremely complex to manage. Hence the method adopted for WAVE is stochastic. Whenever any individual device is transmitting on the channel, all other transmitters must wait for an opportunity for the channel to become available before transmitting. The delay before re-transmitting is established on a pseudo-random basis, computed by each transmitter to avoid scheduling its next attempt at precisely the same time as other transmitters experiencing the same unavailability of the channel.

Called Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA), this scheduling technique has been shown to be perfectly reliable in many familiar traffic scenarios. For instance, at a signalized intersection of two city streets with a relatively small number of vehicles in the vicinity, it is simple to ensure that any vehicle can be warned of an impending T-collision with another unseen vehicle, obscured by buildings and approaching at high speed in the transversal direction.

On the other hand, urban freeways, where the application of V2V has the greatest potential benefit, also create the greatest potential for degradation in service, particularly at peak hour traffic volumes when delays in delivery of messages can exceed the intervals at which the messages need to be sent in order to fulfill the needs of the safety mechanisms which they support.

Simulations indicate that the delays begin to exceed acceptable levels at a volume of 1,000 messages, or heartbeats, per second. As the current SAE J2735 specification requires a frequency of 10 heartbeats per second per vehicle, anything over 100 vehicles within range of each other means heartbeats cannot all be delivered with reliability. A hundred vehicles correspond to the traffic density on a 5-lane highway with vehicles travelling at roughly 60 MPH and maintaining safe stopping distances, within a 420-metre zone.

The need for more channels

The solution lies in the recognition that, beyond the single channel on which heartbeat broadcasts are required by the specification, there are at least four other channels available within the WAVE frequency band that can be assigned to delivering heartbeats, and allocated to different directional segments of freeways so as to avoid excessive use of any single channel within a specified geographic perimeter.

Until now, testing has focused primarily on accurately communicating between several vehicles under controlled conditions (such as the Ann Arbor, Michigan city test area). It is only recently that the industry has begun to focus on extremely high congestion areas and times and the resulting problems with channel contention. 

(For more on V2V standards in North America, see Ann Arbor and the future of V2V/V2I, part I and Ann Arbor and the future of V2V/V2I, part II.)

The design of the WAVE standards has been aimed at meeting the primary objective of improved road safety while simultaneously supporting less time-critical applications which create a wealth of new business opportunities for both the automakers as well as aftermarket service providers. The solution proposed for using multiple channels to support V2V will diminish the availability of these channels for other purposes, thereby reducing, but not eliminating, their potential commercial value.

But all this is manageable within the WAVE framework. The flexibility of the WAVE design allows decision makers to adjust the allocation of RF bandwidth so that the primary safety objective is not compromised by the limitations of the technology under conditions of heavy vehicular traffic volume. But it also makes it possible to maintain the support of services that will attract the investment required from both governments and business for full and rapid deployment of WAVE.

Martin Nathanson is the founder and CTO of Paxgrid Telemetric Systems.

For all the latest telematics trends, check out Telematics Munich 2013 on Nov. 11-12 in Munich, Germany, Telematics for Fleet Management USA 2013 on Nov. 20-21 in Atlanta, Georgia, Content and Apps for Automotive USA 2013 on Dec. 11-12 in San Francisco, Consumer Telematics Show 2014 on Jan. 6 in Las Vegas, Telematics for Fleet Management Europe 2014 on March 12-13 in Amsterdam, The Netherlands, and Content and Apps for Automotive Europe 2014 on April 8-9 in Munich, Germany.

For exclusive telematics business analysis and insight, check out TU’s reports: Telematics Connectivity Strategies Report 2013The Automotive HMI Report 2013Insurance Telematics Report 2013 and Fleet & Asset Management Report 2012.

 


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