Two major problems afflict automobile-based transportation systems in the U.S. and around the world: traffic accidents and congestion. About half a million people are killed each year in traffic accidents worldwide. While Americans make up only 3% of the world's population, they account for almost 9% of those traffic fatalities. In 2003, more than 42,000 Americans died in traffic-related accidents. Similarly, 42,000 or more died the year before that.¹ In fact, the total number of traffic deaths in the U.S. hasn't changed significantly in 10 years.² For every fatality, there are approximately 40 injuries that flood hospitals and drive up medical costs. Traffic accidents cost the American economy more than $230.6 billion.

The resulting legal costs and insurance rates escalate faster than the rate of inflation. Recently, a beer company was sued for a traffic accident caused by a drunk driver. Because the beer company had made drinking so attractive, the suit contended, it made the individual lose control of his common sense. It's not a stretch to think that automobile firms could be sued for building vehicles that are capable of excessive speeds and producing advertisements that make reckless driving attractive to inexperienced automobile operators.

The second major automobile-transportation problem is that urban and suburban roads are becoming clogged with vehicles. From 1950 to 1986, the U.S. population increased by 60%. Yet the number of automobiles grew by 257%. During this same time period, new road and highway construction declined. Even if funding was obtained to help alleviate the current congestion, major highway improvements can take 10 to 15 years to complete. The mismatch in demand and resources has significantly increased commuter delays.

For an example, look at the Hollywood Freeway. It was built in 1965 to handle 120,000 cars a day. By 1970, almost twice that volume clogged the road. Rush-hour traffic in Los Angeles crawls along at 35 mph. If no improvements are made, traffic will slow to 11 mph by 2010. According to a Federal Highway Administration study, recurring congestion along urban freeways during 1987 caused 700 million vehicle hours of delay. Non-recurring congestion resulted in over 1.2 million vehicle hours of delay. The costs of national traffic congestion—including lost productivity and accidents—are estimated at $100 billion annually.

Current safety features, such as airbags and reinforced frames, attempt to limit an accident's impact. But they don't prevent the accident from occurring. Many proposed accident-avoidance solutions involve expensive, complex technologies like vehicle-mounted radar or highly automated highway systems. A major concern with these systems is the complexity that's inherent in the designs. That complexity makes it likely that these solutions will only be used in the distant future—if ever.

A more practical solution could be found in a differential Global Positioning System (DGPS)/wireless-mesh system (FIG. 1). This solution uses an inexpensive networked system that is redundant and distributed. The system is therefore more reliable than alternate designs. It also uses off-the-shelf elements, which can be easily integrated into existing vehicles.

Metcalfe's Law drives the DGPS/wireless-mesh design. Bob Metcalfe, the inventor of Ethernet, hypothesized that the value of a network increases along with the number of nodes that are attached to it. Obviously, the value of the network also depends on the information that is passed among the nodes (i.e., the amount of intelligence provided by the node). Linking to a thousand nodes wouldn't be useful if the information dealt with car care and you were tasked with designing a jet engine. The DGPS/wireless-mesh system enhances the value of the network by providing information on the vehicle state (for example, location) to improve safety and traffic flow.

A number of wireless networks in development—all of various frequencies and bandwidths—could satisfy the communications needs of this system. The IEEE provides several Wi-Fi standards, which are generically lumped under 802.11x. Equipment that complies with those Wi-Fi standards is available from a number of sources. Yet research has shown that the performance of a wireless system varies under different vehicle speeds, traffic conditions, and driving areas.³

Mesh networks are an alternative to communication systems that have a commanding transmitter and numerous receivers. A mesh network has no central controlling element. All of the nodes in the mesh system can directly or indirectly communicate by relaying or "hopping" data through intermittent nodes (FIG. 2). This scheme provides a number of benefits over the networks that are based on one central transmitter.

The mesh networks have an important advantage over centralized networks, as the Federal Communications Commission (FCC) regulations limit the maximum transmission power of those networks. In a centralized network, each transmitting node emits with enough power to reach any other node in the network with one transmission. When two nodes transmit at the same time, channel contention limits capacity. It also causes problems in environments like urban areas, which have a great deal of high-bandwidth demands. Typical frequency, time, or coding schemes (or a combination of the three) are used to segment the channel into subchannels. N nodes can transmit without contention when the channel is divided into N subchannels. As a result, each subchannel has 1/Nth the capacity of the full channel.

In a mesh network, a node only needs to transmit with enough power to reach adjacent nodes. These nearby nodes will forward the data to more distant nodes. The process is continued until the data is received by the destination node.

In addition to providing spatial separation, the low-transmission power of a mesh network supports higher bandwidth than the centralized system. The signal-to-noise ratio (SNR) declines in a mesh network because the number of errors at the receiver increases as transmission distance increases (assuming a constant power level). To transmit over longer distances, wireless devices
use variable forward-error-correction encoding schemes or less complex modulation approaches. These schemes result in channel capacity that decreases as the transmission distance increases.

To avoid this problem, mesh networking transmits data over several short hops instead of one long hop. By using lower transmit power than centralized networks, the mesh-network nodes that are positioned at different geographic areas can transmit simultaneously without interfering with each other. The potential exists for more than N devices to simultaneously transmit within a mesh network without contention.