Four people are killed in motor vehicle accidents in the US every hour.  Based on 2007 information from the National Association of Commissioners of Insurance and 2008 information from the United States Department of Transportation (DOT), the cost of vehicle insurance in the US in 2008 was $201 billion. 
Consumer Reports magazine in 2012 reported an additional $99 billion dollars in medical costs and lost time due to vehicle accidents every year in the US. 
Thus, the cost of vehicle accidents in the US is approximately $300 billion per year.  This is approximately $1000 for every US resident every year. 
Various technology-based methods have been proposed to reduce the number of vehicle accidents.  The basis of some of these methods is wireless transmission by a sending vehicle of its position and speed, then the computation by a receiving vehicle of a possible collision between the transmitting vehicle and the receiving vehicle by computing the future positions of both vehicle based on the received information combined with the position and speed information of the receiving vehicle.  Then, either the driver of the receiving vehicle is warned to take evasive action or evasive action is initiated by the receiving vehicle automatically. 
Such systems are sometimes called “V2V” for Vehicle-to-Vehicle communication. 
V2V systems have been deployed on a limited basis for commercial trucks and pilot tests have been performed on automobiles.  However, such systems are not in widespread use, nor is widespread use being implemented or planned.  A collision detection system for ships is currently widely used. 
A standard has been developed and adopted for V2V communication by IEEE: IEEE 802.11p.  This is not the protocol used by the existing ship-to-ship collision detection system. 
These systems as proposed and developed suffer from serious weaknesses.  One weakness is unnecessary complexity.  This complexity hinders development speed and adds cost, which further delays deployment. 
Another, even more serious weakness, is that the proposed systems will not in fact be effective at significantly reducing accidents for many years.  Current systems require BOTH the transmitting vehicle and the receiving vehicle to be equipped with compatible V2V devices.  The US DOT estimates in 2012 that if ALL vehicles were equipped that the accident rate would be reduced by 50%.  Thus, if 25% percent of all vehicles were equipped with a V2V system, 25%*25%*50%, or a 3% reduction in accident rate would be achieved.  If vehicle accidents cost on average $1000 per year per person, the net dollar advantage per person is only $30, which is far below the currently expected cost per vehicle of equipping a vehicle.  Even reaching a 25% installed density of V2V systems will take many years, assuming current trends on new vehicle purchases.  The average age of vehicles in the US is 11 years.  If 50% of new vehicle buyers purchase with an installed V2V, then after 11 years the penetration percentage is approximately 25%.  Thus, with the V2V systems currently proposed, there will not be sufficient motivation by either buyers to purchase optional V2V systems, or for the government to mandate required V2V systems. 
This calculated low effectiveness of proposed systems understates the problem.  In fact, a higher proportion of accidents are caused by older vehicles than new vehicles.  Also, for early buyers, the effectiveness is even less than the eventual 3%.  Thus, equipping only new vehicles is even less effective that the uniform distribution assumed in the above calculations. 
Another serious weakness of V2V systems as proposed is the use of an inappropriate, non-deterministic basis for message transmission.  Real-time systems, particularly those related to safety, as is V2V by its very definition, require deterministic, consistent delivery of information.  The systems as proposed use non-deterministic, “random back-off” transmission of messages, such as CSMA/CA.  Such non-deterministic systems were designed for, and are appropriate for, non-real time applications such as loading web pages and sending text messages. 
Yet another serious weakness of V2V systems as proposed is lack of a simple, usable priority system that is integrated with bandwidth allocation.  Priority of messages is important to assure that the most important messages get through while the least priority messages are delayed or dropped. 
Yet another serious weakness of V2V systems as proposed is lack of clear distinction between emergency vehicle messages and non-emergency vehicle messages. 
Yet another serious weakness of V2V systems as proposed is lack of clear bandwidth allocation rules separating safety-related messages from non-safety related messages. 
Yet another serious weakness of V2V systems as proposed is lack of dynamic ability to calibrate and reduce location errors between different vehicles. 
Yet another serious weakness of V2V systems as proposed is the lack of ability to retransmit messages in a relay.  A message relay allows messages to reach beyond the immediate transmit range. 
Yet another serious weakness of V2V systems as proposed is the lack of ability to send “courtesy” messages.  Such messages significantly increase the value of an installed system to a driver, and thus increase the installed penetration rate. 
Yet another serious weakness of V2V systems as proposed is a lack of ability to practically include pedestrians and bicycles in the system. 
Yet another serious weakness of V2V systems as proposed is a lack of ability to take advantage of widely popular personal, mobile electronic devices to increase the installed penetration rate. 
Yet another serious weakness of V2V systems as proposed is a method to limit transmission power; or a method to limit range. 
Yet another serious weakness of V2V systems as proposed is lack of a complete application layer protocol, such as message formats and meanings.  Without this specification there is no compatibility between different manufacturers or implementations. 
Yet another serious weakness of V2V systems as proposed is lack of roadway lane information.  Such lane information is highly desirable for an effective V2V system.