Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa

Potential Safety Benefits of Intelligent Transportation System (ITS) Technologies

Howard Preston

BRW, Inc.,
700 Third Street South,
Minneapolis, Minnesota 55415.

This paper presents observations about the potential safety benefits associated with the implementation of intelligent transportation system technologies. Information will be presented that documents the results of previous research, identifies additional opportunities, describes two operational tests that will begin this summer, and then provide a summary of the potential safety benefits.

Results of Previous Research

BRW, Inc. has previously participated in two research projects for the Federal Highway Administration that focused on the potential safety benefits associated with advanced transportation technologies. Both of these studies were part of the Automated Highway System (AHS) initiative. The initial phase or Precursor Systems Analysis (PSA) identified safety characteristics on both urban and rural freeways. The second phase was part of the Automated Highway System Consortium and consisted of evaluating the potential safety benefits associated with the deployment of Honeywell's AHS concept.

The work for the PSA consisted of first dividing Minnesota's freeway system into rural and urban segments based on traffic flow and design characteristics and then documenting the specific safety characteristics associated with each segment based on Minnesota Department of Transportation (MnDOT) accident records. Three years of accident records and almost 5,200 accidents were reviewed. The results of this effort are summarized in Table 1 and indicate that there is a substantial difference in rural and urban freeway accident patterns.

Table 1 Distribution by Type of Accident on Rural and Urban Freeways
Type of Accident Rural Urban
Rear End 12.9% 50.5%
Sideswipe 7.3% 17.4%
Right Angle 8.4% 2.4%
Head On 1.5% 0.8%
Ran Off Road 33.7% 17.8%
Hit Deer 25.1% 0.4%
Other 11.1% 10.7%
The most common types of accidents on Minnesota's rural freeways were single vehicles running off the road or hitting deer, which accounted for almost 60% of all of the accidents. In contrast, the most common types of accidents on urban freeways were multiple vehicle accidents, either rear end or sideswipes, which accounted for almost 70% of all of the accidents.

Following the documentation of the distribution of the types of rural and urban freeway accidents, a traffic safety analysis was completed that estimated the minimum expected accident reduction based on the deployment of AHS technologies, primarily lane keeping and collision avoidance. The results of this analysis indicated that:

However, in order to put this estimated minimum freeway accident reduction into a statewide perspective, the following points must be considered.

1. Minnesota's freeway system is already the safest type of roadway facility, with accident rates ranging from 40% to 75% less than comparable rural and urban arterial roadways. (See Table 2.)

Table 2 Roadway Accident Rates by Facility Type (Accidents per Million Vehicle Miles)

Rural Urban
2-Lane 1.2 3.7
3-Lane N.A. 5.0
4-Lane Undivided N.A. 6.8
4-Lane Divided N.A. 4.0
5-Lane N.A. 4.0
Expressway 0.9 2.0
Freeway 0.7 1.7

2. The mileage of freeways in Minnesota amounts to less than 10% of all state highway miles and less than 1% of all road mileage in the state. (See Table 3.)

Table 3 Distribution of Roadway Mileage by Type of System
System Mileage Percent of Total
Interstate 900 0.7
State Trunk Highway 11,000 8.5
Local Roads 118,100 90.8
Total Statewide 130,000 100.0

3. The total number of accidents on Minnesota's freeways accounts for approximately 10% of the total annual number of accidents statewide. (See Table 4.)

Table 4 Distribution of Accidents in Minnesota by Roadway System
System Number of Accidents Percent of Total
Freeway 10,122 10.2
State, County & Local 89,579 89.8
Total Statewide 99,701 100.0

4. The reduction of 1,300 freeway accidents per year amounts to only about a 1% reduction in total accidents statewide.

5. Even a 100% reduction in freeway accidents would still only result in an overall 10% statewide accident reduction.

Therefore, it was concluded that in order to achieve the maximum safety potential of intelligent transportation initiatives, the technologies have to be expanded beyond freeways to the state, county, and local road systems.

Additional Safety Opportunities

In order to identify additional safety opportunities off of the state's freeway system, the accident records for the state's system of rural and urban arterials were analyzed. The results of this effort indicated three key points. First, accident frequency is almost always a function of exposure. However, 70% of the vehicle miles of travel in Minnesota is in urban areas and 70% of the fatal accidents are in rural areas. Additional analysis of the distribution of types of accidents indicated that single vehicles hitting deer and running off of the road were the most common types of rural accidents and that multiple vehicle rear end and right angle were the most common types of urban accidents, accounting for almost 50% of the total accidents, respectively. A review of the fatal accident records indicated that single vehicle run off the road and multiple vehicle right angle and head on accidents account for more than 75% of all fatal non-freeway accidents statewide.

Further analysis of data from the National Highway Traffic Safety Administration's Fatal Accident Reporting System found that in Minnesota fatal accidents there is a substantial difference in emergency medical service response times in rural and urban areas. These differences are summarized in Table 5 and indicate that total response times in rural areas are on the average 60% greater than in urban areas.

Table 5 Emergency Medical Service Response Times (Minutes)

Rural Urban
Crash to EMS Notification 7 3
EMS Notification to EMS Arrival 11 6
EMS Arrival to Hospital Arrival 30 22
Total Time to Hospital Arrival 46 29

It appears reasonable to assume that the greatest opportunities to improve safety off of the freeway system would involve efforts focused on the following areas:

Based on this analysis it appears that a program that addresses single vehicle run off the road accidents on rural roadways, multiple vehicle rear end and right angle collisions on urban arterials, and an automated mayday calling system for emergency medical services would likely have a very positive impact on traffic safety by reducing both the absolute number and severity of accidents.

Operational Tests

BRW, Inc. is assisting the Minnesota Department of Transportation with the deployment of two operational tests during the summer of 1996. Both tests involve the use of optical vehicle detection and changeable message signs. The first test will be in an urban area at a freeway lane drop at an interchange with a state highway that is one of the most heavily traveled routes to recreational areas in central Minnesota. Congestion along this state highway has occasionally queued traffic back down onto the Interstate in the past, and scheduled reconstruction of the state highway this summer is expected to make traffic conditions substantially worse. The congestion has resulted in some rear end accidents in the lane drop, and the 60 mile per hour speed differential between the lane drop and the adjacent through lanes presents an opportunity for more serious accidents. Therefore, since there was no practical way to eliminate the congestion due to the reconstruction, it was decided to deploy a dynamic queue monitoring and advance warning system. This system consists of the optical vehicle detectors to identify the formation and the length of the queues in the lane drop, static warning signs with wig-wag flashers, and a changeable message sign placed in advance of the end of the queue to warn drivers of the unexpected stopped condition ahead. The system will be designed so that the flashers and the changeable message sign will be operational only if there is stopped traffic in the lane drop.

The second test will be located in an as yet undetermined rural location. The test is based on previous research that has indicated that a higher percentage of run off the road accidents occur in and near horizontal curves and that the single best predictor of whether on not a vehicle will successfully navigate a curve is the vehicle's speed. The purpose of the test is therefore to determine if an optical vehicle detector can correctly determine vehicle speeds on the approach to a horizontal curve, compare the calculated speed to a predetermined safe speed, and then if necessary activate a special warning on a changeable message sign. Several weeks of video taped data will be analyzed to determine if the system can correctly determine vehicle speeds and if the deployment has any impact on the percentage of vehicles that successfully navigate the curve.


CTRE is an Iowa State University center, administered by the Institute for Transportation.

Address: 2711 S. Loop Drive, Suite 4700, Ames, IA 50010-8664

Phone: 515-294-8103
FAX: 515-294-0467


Iowa State University--Becoming the Best