ISSUES AND RECENT TRENDS IN VEHICLE SAFETY COMMUNICATION SYSTEMS

S. TSUGAWA

ISSUES AND RECENT TRENDS IN VEHICLE SAFETYCOMMUNICATION SYSTEMS
Sadayuki TSUGAWA
Professor, Department of Information Engineering
Faculty of Science & Technology
Meijo University, Aichi, Japan(Received February 10, 2005)
This paper surveys the research on the applications of inter-vehicle communications, the issues of the deployment and technology, and thecurrent status of inter-vehicle communications projects in Europe, the United States and Japan. The inter-vehicle communications, defined here ascommunications between on-board ITS computers, improve road traffic safety and efficiency by expanding the horizon of the drivers and on-board
sensors. One of the earliest studies on inter-vehicle communications began in Japan in the early 1980s. The inter-vehicle communications play anessential role in automated platooning and cooperative driving systems developed since the 1990’s by enabling vehicles to obtain data that would be
difficult or impossible to measure with on-board sensors. During these years, interest in applications for inter-vehicle communications increased in theEU, the US and Japan, resulting in many national vehicle safety communications projects such as CarTALK2000 in the EU and VSCC in the US. The
technological issues include protocol and communications media. Experiments employ various kinds of protocols and typically use infrared, microwave or millimeter wave media. The situation is ready for standardization. The deployment strategy is another issue. To be feasible, deployment
should begin with multiple rather than single services that would work even at a low penetration rate of the communication equipment. In addition,
non-technological, legal and institutional issues remained unsolved. Although inter-vehicle communications involve many issues, such applications
should be promoted because they will lead to safer and more efficient automobile traffic.
Key Words: ITS (Intelligent Transport Systems), Inter-vehicle communications, AVCSS (Advanced Vehicle Control and Safety Systems), Driver assistance systems, Automated driving systems

1. INTRODUCTION
The Intelligent Transport Systems (ITS) attempt toprovide a solution to the twentieth-century’s negative
legacy of traffic accidents and congestion. Large-scalenational ITS projects began in Europe, the US and Japanin the mid-1980s but, compared to those early years, recent ITS efforts have seen a markedly greater focus onsafety. The EU’s eSafety initiative1, announced in November 2002, aims to halve the number of annual traffic
accident fatalities in the EU by 2010. The United States’
ITS Ten-Year Plan2, announced in January 2002, talks
about achieving a 15% reduction in the number of annual traffic fatalities by 2011, while the National Intelligent Vehicle Initiative (NIVI) Meeting in June 2003
announced a goal of reducing the rate of traffic accident
fatalities from the then-current 1.51 deaths per 100 million vehicle-miles traveled (VMT) to 1 death per 100 million VMT by 2007. Against this background of ITS
activities, vehicle safety communication systems based oninter-vehicle communications have been a topic of greatinterest around the world in recent years. Taking up thissubject, this paper addresses inter-vehicle communications technologies, the history of systems for the applications, and current trends, and some issues for the future.

The term of “inter-vehicle communications” can be also
expressed as “vehicle to vehicle communications” or “carto car communications,” and “inter-vehicle communications” is used in this paper.

2. ITS AND INTER-VEHICLECOMMUNICATIONS
2.1

The position of inter-vehicle communications
within ITS
Under the ITS system architecture of the US and
3
Japan , as indicated in Figure 1, ITS communications systems are defined to include four types of communications
systems: inter-vehicle (vehicle to vehicle), road-to-vehicle, wide-area wireless and wireline. As this makes evident, the inter-vehicle communications are an importantelement of the ITS field. By definition, inter-vehicle communications include communications between drivers andbetween vehicles, but we will define it as communications
between on-board computers. Systems introduced in thispaper are accordingly those based on the definition.
One characteristic of inter-vehicle communications
is that unlike road-to-vehicle communications, where thecommunications site is determined by the location of road
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THE COMPUTERIZATION OF TRANSPORTATION: Sophisticated Systems Incorporating IT in the Mobility of People and Goods

Travelers

Vehicle to Vehicle Communications

Emergency
Management

EmissionsManagement

Wide Area Wireless(Mobile) Communication

VehicleTransit
VehicleCommercial
VehicleEmergency
VehicleToll
Administration

TransitManagement

Commercial
VehicleAdministration
Fleet andFreight
Management

Information
Service
ProviderArchived
DataManagement

Wireline (Fixed-Point to Fixed-Point) Communication

Dedicated Short Range
Communications

Traffic
Management

Personal
Information
Access

Vehicle3. APPLIED SYSTEMS INCORPORATINGINTER-VEHICLE COMMUNICATIONS

Centers

RemoteTraveler
SupportRoadwayToll
Collection
Parking
Management

Roadside

Commercial
Vehicle Check

Fig. 1 Communication systems in ITS system architecture

beacons, communications with other vehicles can takeplace anywhere. Consequently, information and data thatwould be difficult or impossible to measure from a single
vehicle can be collected through inter-vehicle communications. Inter-vehicle communications make it possible toexpand the horizon of on-board sensing systems, a characteristic exploited in vehicle safety communication systems.
2.2

The history of research into inter-vehicle communications
Probably the earliest research into inter-vehiclecommunications was conducted in the early 1980s at the
Association of Electronic Technology for Automobile
Traffic and Driving (now the Japan Automobile ResearchInstitute). This research treated inter-vehicle communications primarily as traffic and driver information systems
incorporated in ATMS/ATIS (Advanced Traffic Management Systems/ Advanced Traveler Information Systems).
Efforts have since been made to apply it to driver assistance and automated driving systems incorporated in
AVCSS (Advanced Vehicle Control and Safety Systems).
From the 1990s through 2000, automated platooning systems such as California’s PATH (Partners for Advanced
Transit and Highways)4 and the EU’s Chauffeur5, as wellas cooperative driving systems in Japan6-8, made automated vehicle platooning a reality through the transmission of data, such as each vehicle’s acceleration, throughthe inter-vehicle communications that would be impossible to measure from other vehicles. Recently, as described above, systems for improving automobile traffic
safety that use inter-vehicle or road-to-vehicle communications to relay incident or emergency information from a
preceding vehicle to succeeding vehicles9,10 have generated a great deal of interest in Europe, the US and Japan.

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Broadly speaking, inter-vehicle communications areapplied to traffic and driver information systems within
ATMS/ATIS (Advanced Traffic Management Systems/
Advanced Traveler Information Systems) or to driver assistance and automated driving systems within AVCSS(Advanced Vehicle Control and Safety Systems). In general, inter-vehicle communications in ATMS/ATIS applications are ad hoc and sporadic. This is also usually thecase with many driver assistance applications in AVCSS.
For automated driving systems such as automated platooning inter-vehicle communications must be maintainedwith a short period. Major research to date and its distinctive characteristics will be introduced below. Note that
ATMS, ATIS and AVCSS are technical terms defined by
ITS America11 and represent the mainstream ITS systems.
3.1

ATMS/ATIS applications
Around 2000, a system for incident warnings wasdeveloped by the Ohio State University using 220MHz
VHF10, whose range of 1 to 2 kilometers made the intervehicle communications possible even where the penetration rate of the communication equipment is low. The
researchers were interested in inter-vehicle communications because of the difficulty of preparing infrastructure
intelligence to serve such a vast country.
Also around 2000, a German consortium proposeda system for relaying accident and incident information
over inter-vehicle communications using the same
800MHz frequency band to that used for car phones9. The
idea was that sending accident or incident information to
succeeding vehicles would prevent multiple-vehicle accidents, particularly on suburban roads under poor visibility conditions. Accidents or incidents would be detected
automatically based on the use of hazard lights, vehicle
acceleration, airbag deployment or signals from the antilock braking system (ABS) and sent, together with GPS(Global Positioning System) location data from the navigation system, to be sorted out on the receiving end.
3.2

AVCSS applications
In the United State, Demo97, a large-scale demonstration of an automated driving system, was held in San
Diego in 1997. The California PATH team4 participating the Demo had a platoon of 8 passenger vehicles demonstrating automated driving at a speed of 96km/h and agap distance of 6.3m, as illustrated in Figure 2. This sys-

ISSUES AND RECENT TRENDS IN VEHICLE SAFETY COMMUNICATION SYSTEMS

tem used inter-vehicle communications in a 900MHz
band using off-the-shelf wireless LAN (Local Area Network) equipment to transmit and receive acceleration anddeceleration information from preceding vehicles andcommands from the lead vehicle. The data transmission
rate was 122kbps.
Around 1997, California PATH conducted tests ofinter-vehicle communications using infrared. As illustrated in Figure 3, the system employed transmitting andreceiving equipment attached to the rear bumper of thepreceding vehicle and the front bumper of the succeeding
vehicle, and was distinguished by how it varied transmission rate with the bit error rate (BER). It was possible toraise the transmission rate as the BER decreased at shorter
distances between the vehicles. This characteristic meetsthe need for more precise control as vehicles move closer
together, which leads to higher transmission rate.
The European ITS project known as PROMETHEUS(PROgraMme for a European Traffic with Highest Efficiency and Unprecedented Safety), which ran from 1987
until 1994, developed an inter-vehicle communications
system using 57GHz to achieve cooperative driving
among multiple vehicles. Figure 4 shows the communication equipment attached to the vehicles.

S. TSUGAWA

The EU’s Chauffer project5 developed the system
for truck platooning shown in Figure 5. The initial system
involved inter-vehicle communications of acceleration,
deceleration and other data from preceding vehicle to succeeding vehicle, employed a 2.4GHz band, and the communication period was 40msec and the rate was 230kbps.
The later version of the system employed a 5.8GHz band.
The Netherlands Organization for Applied Scientific Research (TNO) incorporated inter-vehicle communications using infrared in the MENSOR system developed
beginning in 1998. As illustrated in Figure 6, three transmitters attached to the rear bumper of a preceding vehiclecommunicated with one receiver attached to the front grillof a succeeding vehicle. The goal was to smooth traffic
flow and prevent congestion by sending braking information from a preceding vehicle to a succeeding vehicle
that would prevent rear-end collisions. The system subsequently developed into the EU’s CarTALK2000 project
discussed below.
An noted before, Japan was at the leading edge ofthe inter-vehicle communications research, beginning before either Europe or the US with research conducted by
the author and the Association of Electronic Technology
for Automobile Traffic and Driving. In the late 1980s, theauthor and his colleagues employed infrared signals to

Fig. 4 Inter-vehicle communication unit developed
during PROMETHEUS project in 1994

Fig. 2 Platooning by California PATH during AHS demo
in 1997

Fig. 3 Inter-vehicle communications with infrared by
California PATH in 1997

Fig. 5 Heavy duty trucks for Chauffeur in 1998
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perform inter-vehicle communications between automated guided vehicles for factories, developing vehicles
that followed each other based on “soft links12.” This was
one of the earliest systems to apply the inter-vehicle communications to vehicle control.
In 1997 the Association of Electronic Technology
for Automobile Traffic and Driving conducted cooperative driving system tests using infrared signals for intervehicle communications (Phase I)6. Under cooperative
driving, the following distance between vehicles wasmeasured by the triangulation between a pair of infraredmarkers on the roof of the vehicles, one of which functioned as a transmitter. Figure 7 shows four test vehicles,
capable of both inter-vehicle communications and of vehicle gap distance measurement, assisting drivers by indicating when to merge.
Later, in 2000, the author and the Association ofElectronic Technology for Automobile Traffic and Driving conducted new cooperative driving system tests(Phase II)7,8. This cooperative driving system incorporated the inter-vehicle communications among five automated driving vehicles using 5.8GHz DSRC (Dedicated
Short Range Communication). As illustrated in Figure 8,
the group of automated driving vehicles formed a flexible platoon enabling merging and lane changing. Eachvehicle was equipped with D-GPS to measure the present
vehicle location, and transmitted the location, speed, direction of the vehicle and the presence of obstacles, if any,through the inter-vehicle communications at a commu-

nication period of 20ms.
Research using inter-vehicle communications indriver assistance systems for blind intersections has also
been conducted in Japan13-15. Sending the vehicle position data by GPS through the inter-vehicle communications enables other drivers to be aware of approachingvehicles that are beyond their field of view. The systems
developed by Keio University13 and by the author and hiscolleagues14 employed 5.8GHz DSRC. Figure 9 illustrates the author’s system being tested. One thing thatmust be noted with such systems is that over-reliance on
the inter-vehicle communications by drivers of equipped
vehicles raised the possibility of creating dangers for nonequipped vehicles.

Fig. 6 Inter-vehicle communications with infrared for
CarTALK2000 in 2001

Fig. 8 Cooperative driving with 5 automated vehicles
linked by inter-vehicle communications in 2000

Fig. 7 Driver assistance by inter-vehicle communications with infrared in 1997

Fig. 9 Collision avoidance at a blind crossing withinter-vehicle communications in 2001

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3.3

Infotainment applications
Efforts have also been made to transmit movie data
among vehicles using inter-vehicle communications. The
Communication Research Laboratory in Japan conducted
research on inter-vehicle communications using 60GHz
millimeter waves16. Using millimeter waves for line-ofsight communications, experiments shown in Figure 10
were conducted at a relatively short gap distance for highquality TV picture transmission.
The author and his colleagues conducted experiments in transmitting movie data through inter-vehiclecommunications using 5.8GHz DSRC. The objectives ofthis system, shown in Figure 11, were to prevent accidents
and congestion by transmitting traffic condition informa-

ISSUES AND RECENT TRENDS IN VEHICLE SAFETY COMMUNICATION SYSTEMS

tion in a movie data format from preceding vehicles tosucceeding vehicles17. Transmission of in-vehicle scenes
among partner vehicles allows creation of a “virtual cabin”
of the partner vehicle in one’s own vehicle, with possibilities for inter-vehicle communication entertainment.

S. TSUGAWA

Trends in Europe
Table 1 lists European (EU) vehicle safety communication projects. ITS in the EU is characterized by a relatively lower emphasis on infrastructure-side intelligence

and a correspondingly higher level of interest in inter-vehicle communications. The objectives and characteristics
of each project are addressed below.
The joint German-French IVHW (Inter-Vehicle
Hazard Warning) project aims to realize the functions
listed in Table 2. The frequency band used for the intervehicle communications in IVHW is 869.4MHz - 869.65
MHz with a potential range of as much as 1km on urban
roads (reliable range of about 500m) and 2km on highways (reliable range of about 1km). The GPS data is included in the communication content and the information
is sorted at the receiving end with the localization data.
The German FleetNet project, in which automobile
manufacturers, communications companies, universities
and research organizations took part, aimed to create acommunications platform for inter-vehicle communications. Applications were varied, including cooperative
driving assistance, decentralized floating cars and Internet
access, and were demonstrated with six small passenger
cars. One of the fruits of the project was a communication network routing algorithm based on vehicle position
that produced better results than other ad hoc routing algorithms. The NOW (Network on Wheels) project devel-

Fig. 10 TV image transmission over inter-vehicle communications with millimeter wave in 2001

Fig. 11 Movie transmission over inter-vehicle communications with 5.8GHz DSRC in 2004

4. TRENDS IN VEHICLE SAFETYCOMMUNICATION SYSTEMS IN EUROPE,THE US AND JAPANAs described in the previous section, research in theapplications of inter-vehicle communications to ITS began in the 1990s, but the conspicuous trend in inter-vehicle communications research in Europe, the US andJapan since the start of the twenty-first century has beena focus on safety.
4.1

Table 1 European vehicle safety communication projects
Project

PeriodSponsor Countries

Objectives

IVHW(Inter-Vehicle Hazard Warning)

2000-2002

Germany, France

Hazard Warning

FleetNet

2000-2003

Germany

Communication Platform

CarTALK2000

2001-2004

EU

Cooperative Driving in Practice

INVENT VLA(Intelligenter Verkehr undNutzergerechte Technik, Verkehrs Leistungs Assistenz)

2001-2005

Germany

Traffic Efficiency,
Cooperative Driving

WILLWARN(Part of the PReVENT project)

2004-2006

EU

Hazard Warning

NOW(Network on Wheels)

2004-2007

Germany

Demonstration and Standardization

C2C-CC(Car to Car Communication Consortium)

2001-

Consortium of
Private Companies

Standardization

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Table 2 Functions included in IVHW
Function

Description

Accident and congestion warning

Transmission of accident or congestion information from preceding vehicles,
including GPS positioning data.

DTR (Decentralized Traffic Routing)

Each vehicle transmits its own speed, generating a pool of road network traffic
information that makes route guidance possible.

Platooning

Convoying of multiple vehicles.

Lane change assistance

Indicates that a vehicle in the neighborhood will change lanes.

Road surface condition monitoring

Preceding vehicles send road surface information such as coefficient of friction
to succeeding vehicles.

Transmission of traffic conditions ahead

Transmission of road scene information ahead; the view of the road scene isblocked by other vehicles in front.

Identification of the last vehicle in the congestion

Specify the last vehicle in the congestion through transmissions from it tosucceeding vehicles.

Intersection assistancePrevent collisions through communications among intersecting vehicles.

Merging assistance

Assist vehicles in merging smoothing from the on-ramp to the through traffic
lane.

oped from FleetNet.
CarTALK2000 was established within the EU’s
ADASE2 (Advanced Driver Assistance Systems Europe)
ITS project and included the participation of automobile
manufacturers, automobile parts manufacturers, communication companies, universities, and research organizations. It incorporated three applications: a warning systemthat relays information on accidents ahead, break-downs,
and congestion; a longitudinal control system; and a cooperative driving assistance system that supports merging and weaving. For CarTALK2000, inter-vehiclecommunications used mobile, ad hoc multi-hop networks.
The reasons to use inter-vehicle communications were
defined as:1) The inapplicability of mobile phones to important,time-critical applications.
2) The need to use high-speed data transmission.
3) The ability to do without infrastructure-side networks.
4) The ability to reduce communication costs.
These characteristics apply not only to CarTALK2000
but commonly apply to many inter-vehicle communications applications.
The German INVENT (Intelligenter Verkehr undNutzergerechte Technik) project, aimed at safe driving
assistance and optimizing traffic flow, included both automobile and parts manufacturers. Among its eight subprojects, the VLA (Verkehrs Leistungs Assistenz) dealt
with safety through communications. The VLA is a key
technology for driving assistance systems as it allows
drivers to predict traffic conditions and adjust accordingly. VLA inter-vehicle communications envisioned the

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use of 24GHz UWB (Ultra Wide Band) radar at highspeeds (period: 0.01sec) over narrow ranges (50m) to exchange driver intentions and middle-speed (period: 0.1
sec) middle-range (500m) ad hoc networks for transmission of local traffic conditions.
The EU’s PReVENT project for improving driving
safety included the WILLWARN inter-vehicle communications sub-project. WILLWARN provided dispersed
warning information by detecting hazards from vehicles
and sharing information among vehicles through an inter-vehicle communications network.
In addition to the projects mentioned above, theC2C-CC (Car-to-Car Communication Consortium) wasestablished by European automobile manufacturers tostandardize inter-vehicle communications.
4.2

Trends in the US
The US Department of Transportation in 1998 began the IVI (Intelligent Vehicle Initiative) project withthe goal of improving safe driving. As part of this project,IDS (Intersection Decision Support) systems using thecommunication technologies have been developed in thethree states of California, Minnesota and Virginia. The
California IDS18 not only detects straight oncoming vehicles from the ground but also combines GPS data withinter-vehicle and road-to-vehicle communications data foron-board display of other vehicles’ locations. Media andprotocols used conformed to IEEE802.11a. Figure 12
shows the array of antennas installed on the roof of a test
vehicle.
Meanwhile, CAMP (Crash Avoidance Metrics Partnership), a group founded by automobile manufacturers

ISSUES AND RECENT TRENDS IN VEHICLE SAFETY COMMUNICATION SYSTEMS

in 1995 in cooperation with the Department of Transportation and standardization organizations, supported the
VSCC (Vehicle Safety Communication Consortium) from2002 to 2004. The VSCC included automobile manufacturers from Europe, the US and Japan and proposed asystem applying inter-vehicle and road-to-vehicle communications to active safety. System proposals for the near
future include traffic signal violation warning, curve
speed warning and emergency electronic brake lights.
Middle future proposals include pre-crash sensing (thatprepares for unavoidable collisions), cooperative forward
collision warning, left-turn assistance (in the US), lanechange warning and stop sign assistance.
The US Department of Transportation also began
the VII (Vehicle Infrastructure Integration) program in2004 in cooperation with state departments of transportation and automobile manufacturers. VII considers
applications for inter-vehicle and road-to-vehicle communications to active safety, traffic management and traveler information. Using DSRC for inter-vehicle androad-to-vehicle communications, field operation tests willcontinue from 2005 through 2009 with a decision on introduction slated in 2010.
4.3

Trends in Japan
In Japan, three of the four ministries involved withITS are active in inter-vehicle communications. Since
1999, ITS Forum under the Ministry of Internal Affairs
and Communications has proposed systems using intervehicle communications for ACC (including Stop&Go),
cooperative driving, hazard warning, merging and lanechange warning, intersection and curve collision warning, and communications between and within platoons.
The media used depends on the applications, with millimeter waves used for line-of-sight communications andmicrowaves for omni-directional communications. The

S. TSUGAWA

Ministry of Land, Infrastructure and Transport, in its active safety-focused ASV (Advanced Safety Vehicle)
project, uses communications among heavy duty trucks,
passenger cars, and motorcycles to provide notificationof motorcycles ahead in blind curves or hidden from viewon narrow roads and to assist with right turns (in Japan)
by providing notification of motorcycles traveling straightin the opposing lane. The Ministry of Economy, Tradeand Industry has also been active in efforts aimed at standardization of inter-vehicle communications.

5. ISSUES FOR INTER-VEHICLECOMMUNICATIONSAs described above, there has been active researchand development on the inter-vehicle communications
applications to active safety in Europe, the US and Japan. Nevertheless, there remain a number of important
issues that need to be resolved.
5.1

Protocols
There are two requirements for ITS inter-vehiclecommunications protocols. The first is the flexibility to accommodate both the addition of new vehicles to the network and the withdrawal of existing vehicles. The second,particularly when the goal is vehicle control, is real-timedata transmission. The importance of these requirements
varies by applications. Flexibility is more important in
AMTS/ATIS applications. For AVCSS applications, however, it is critical to achieve both flexibility and real-timedata transmission. The compatibility is not always easy.
In the cooperative driving Phase II project conducted by the author and his colleagues, flexibility wasachieved by using a CSMA (Carrier Sense Multiple Access) technology. Real-time data transmission wasachieved by composing a single vehicle control period offive communication periods, using repetitive communication to reduce the packet collision rate. Figure 13 describes the relationship between the number of successfulcommunications within a single vehicle control period andthe rate of communication success. The figure shows thatwhen setting the BER below 10-3 at least one successfulcommunications are performed within a single vehicle control period, which is equal to five communication periods.
5.2

Fig. 12 Antennas for Intersection Decision Support in
2003

Media
Many media have been used and tried in experiments to date, including VHF, microwaves, millimeter
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THE COMPUTERIZATION OF TRANSPORTATION: Sophisticated Systems Incorporating IT in the Mobility of People and Goods

Average Packet Loss (PL)When Idling
When Running
PL = 10^–3

Number of Successes

Fig. 13 Relationship between the number of successful data transmission and the rate of packet
discardwaves and infrared. The former two are non-directional
while the latter two are directional. Many varieties of microwave have been used in experiments, including 800900MHz, 2.4GHz, wireless LAN, 5.8GHz, and 5.9GHz.
The selection of media must match the application.
5.3

Penetration rate
Services using road-to-vehicle communications canbegin with even a single vehicle equipped with a communication device as long as the infrastructure-side
preparations are in order. Services using inter-vehiclecommunications, however, unlike road-to-vehicle communications services, require multiple vehicles equippedwith communication devices. As previously noted, many
services have been proposed that assume high penetration rate of communication equipment or that all vehicles
are so equipped. For example, the systems discussed
above for preventing accidents at intersections assume
that all vehicles are equipped with communication devices. However, given the possibility of equipment failure, loss of power and radio interference, there is no
guarantee of communications. Systems that assume all
vehicles are equipped are not necessarily realistic. Systems to prevent accidents at intersections may be of assistance to drivers but if drivers become dependent uponthem they could create new dangers in situations where
unequipped vehicles are present. Such systems have astrong possibility of becoming a “two-bladed sword.” In
the intersection collision prevention system mentioned
above, a driver relying solely on the system would haveno way of knowing of the presence of an unequipped vehicle. In order to confirm conditions are safe, the driver
must conduct a visual check and not rely solely on the

14 • IATSS RESEARCH Vol.29 No.1, 2005

system, making the system appear meaningless. At thesame time, such a system would have a role to play inbad weather or at other times when a visual safety checkis difficult, and could contribute to the protection of pedestrians, bicyclists and other vulnerable road users, were
they equipped with communication devices to relay proximity information.
The development of inter-vehicle communications
systems should begin with services that are effective evenin the absence of extensive penetration of communication devices. Such systems should provide advantages toequipped vehicles and, although providing no direct advantages for non-equipped vehicles should avoid creating any disadvantages for them. The author took up asystem for relaying incident information from preceding
vehicles to succeeding vehicles as a means of avoiding
multiple-vehicle accidents and congestion on highways,
conducting a simulation to explore the relationship between the penetration rate of communication equipment
and distance of data transmission. The results, thoughvarying with traffic density, indicates that the necessary
penetration rate of the communication devices would be
10% to 20% in order to be able to relay information backalong a single-lane road19. Results also suggested a penetration rate of 5% to 10% would be sufficient for a twolane roadway, and 3% to 7% for a three-lane roadway.
Given a total of 70 million vehicles in Japan, if itwere possible to use the 6 million ETC (electronic tollcollection) communication units in Japan (as of March
2005) for inter-vehicle communications away from the
ETC toll gates, there would be a penetration rate ofroughly 9%, meaning an incident transmission system using inter-vehicle communications is already feasible. Because it is already possible to identify the location of
one’s own vehicle using car navigation systems, equipment could be switched over from ETC use near ETC tollgates to inter-vehicle communications use at any otherlocations. Although there are minor differences in thecommunication system used for ETC in Japan and the inter-vehicle communications system for cooperative driving developed by the author and his colleagues, both use
5.8GHz DSRC and it is not a technical impossibility to
add inter-vehicle communications functionality to ETC
communication devices.
5.4

Road-to-vehicle communications and inter-vehicle communications
Under the ITS system architecture developed in Japan and the US, road-to-vehicle communications and inter-vehicle communications are defined as separate. The

ISSUES AND RECENT TRENDS IN VEHICLE SAFETY COMMUNICATION SYSTEMS

author, however, believes the former to be a subset of thelatter because if a vehicle equipped with an inter-vehiclecommunication device stops along a road side, it engages
in road-to-vehicle communications. In other words, roadto-vehicle communications can be seen as a special caseof inter-vehicle communications. Regarding the two typesof communications in this way rather than in dependent
two types makes it easier to develop application scenariosof the inter-vehicle communications like the use of ETC
equipment described above.
5.5

Introduction of inter-vehicle communications
systems applications
Many ITS systems share a common “chicken-andegg” problem. Stand-alone systems do not face this problem, of course, but systems incorporating road-to-vehicle
or inter-vehicle communications inevitably face the problem of whether infrastructure intelligence or vehicle intelligence comes first, or of who will be first to equip theircars with inter-vehicle communication devices. The key
to overcoming this problem is a system that offers not asingle purpose or service but multiple purposes and services including safety, infotainment and convenience.

6. CONCLUSION
This paper surveys the research on inter-vehiclecommunications and some outstanding issues. Much research and development work has been done in recent
years but there is still no practical inter-vehicle communications system in operation. The inter-vehicle communications hold the possibility of contributing greatly toautomobile traffic active safety by expanding the horizonof both drivers and of on-board sensing systems. Nevertheless, many unresolved issues remain today including
technical issues such as standardization, development issues such as application systems, and legal and institutional issues. Finding solutions for these issues anddeveloping systems for the applications of inter-vehiclecommunications are critical for the realization of safer
automobile transportation.

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