traffic conflict technique

8/18/2019 Traffic Conflict Technique

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TRITA-INFRA 02-010

ISSN 1651-0216

ISRN KTH/INFRA--02/010-SE

EMV\Etapp1\admin\ Slutrapport\KTH\Trafiksäkerhet\Traffic Conflict Technique Projekt: VV dnr AL 9099:3393

Effektmodeller för vägtrafikanläggningar (EMV)

Traffic Conflict Technique

Historical to current State-of-the-Art

Stockholm September 2001Jeffery Archer

Institutionen för Infrastruktur KTH

Avdelningen för trafik och logistik

Lund Traffic Conflict Techniq ueLund Traffic Conflict Techniq ue

0102030405060708090

100

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Time To Accident (secs)

S p e e d ( k m / h )

FatalSevere

Slight

DamageOnly

AccidentsNear Accidents

Slight Conflicts

Potential Conflicts

Undisturbed Passages

SeriousConflicts}


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Summary

The Traffic Conflict Technique (TCT) is perhaps the most developed indirect measure of

traffic safety. The technique itself is grounded in the ability to register the occurrence of near-accidents directly in real-time traffic and therefore offers a faster and, in many respects, morerepresentative way of estimating expected accident frequency and accident outcomes. Themain criticisms levelled at indirect methods concern issues of reliability and validity andwhether accident risk is actually being measured in a satisfactory manner.

History

The conflict technique emanates from research originally at the Detroit General Motorslaboratory in the late 1960’s for identifying safety problems related to vehicle construction(Perkins & Harris, 1968). The use of this technique soon spread to different parts of the world.

TRL in England soon recognised the need to add a subjective scale for observed conflicts asa measure of severity (Spicer, 1973). This technique was based on observer judgementsusing time-lapse filming, thereby proving costly and time-consuming. The Swedish TrafficConflict Technique (STCT) was developed at LTH in different projects during the 1970’s and1980’s before finally reaching it’s present day level of development in 1987 (Hydén, 1987).The Swedish technique focuses on situations where two road-users would have collided hadneither of them made any kind of aversive manoeuvre. The point at which the aversive actionis taken is recorded through observation as the “Time-to-Accident”(TA). The TA valuetogether with the conflicting speed is used to determine whether or not a conflict is “serious”.The technique itself was designed to focus only serious conflicts, although more recent workby Åsa Svensson at LTH has shown that the shape of the severity hierarchy i.e. therelationship between a number of successive severity levels, is also useful for gaining insightinto, and making comparisons between different traffic sites. Other conflict techniques were

also developed parallel to the Lund version. The variance among 8 different existing conflicttechniques was investigated by the International Committee for Traffic Conflict Techniques(now the International Cooperation on Theories and Concepts in Traffic Safety chaired byChrister Hydén) in 1984 (Grayson, 1984). The resulting differences were found to benegligible and mainly dependent on local variations in the definition of severity.

Useabil ity, Reliabili ty and Validity

While the interest for the conflict technique has been considerable, its practical use has beenlimited due to questions concerning reliability and validity and the relative costs involved withdata collection (e.g. Hauer, 1978). The Swedish technique is by far the most commonly used

technique for research purposes, and is recognized in many countries all over the world.Questions concerning reliability are related to the subjective element in the registrationprocess where trained observed judge speed and distance. Tests have shown rates of up to80 per cent agreement between different observers. Questions concerning validity focus onwhether or not the technique is an accurate proximal measure of accident occurrence.

Also, product (predictive) validity concerns the relationship between serious conflicts and thenumbers of accidents. An American study by Migletz et.al. (1985) showed that normal conflictstudies could produce estimates of average accident frequency that were at least as accurateas those based on historical accident data, and Svensson (1992) concluded from Swedishdata that serious conflicts provide a better estimate of the number of expected accidentsinvolving personal injury. The results of studies in many different countries have also beencompared to show that the relative statistics for conflicts and accidents are in agreement

despite environmental differences.


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Another major concern is process validity, i.e. whether the processes that determine conflictsare also the same as those that determine accidents. Hydén (1987) concluded, on the basisof data relating to TA and speed values, that conflicts and accidents did in fact share thesame severity distribution, and that accidents generally had a 0.5 sec lower TA-value andspeeds of up to 10-20 kph than those found in serious conflicts.

State-of-the-Art

A thorough search in the Ingent, a database covering a wide range of different journals in thefields of engineering technology, transportation, social sciences and social and public welfare(e.g. Accident Analysis and Prevention, and Transportation Research) provided surprisinglylittle information. Only four published articles of any interest were found which directly indicatethe use of the traffic conflict technique. The reason for this poor result may perhaps lie in thefact that, although many studies employ the conflict technique as part of their safetyevaluation strategy, few consider it important enough to include in the title or abstract. Asimilarly misfortunate search was also made on the Traffic Psychology section of thePsychInfo database. The articles that were found are listed below:

1) Use of the traffic conflict technique to identify hazardous intersections. Katamine, N. M.; Harmarneh, I. M. Road &transport research, 1998, vol. 7, no. 3, pp. 172) Modeling Conflicts of Heterogeneous Traffic at Urban Uncontrolled Intersections. Rao, V. Trinadha; Rengaraju, V.R. Journal of transportation engineering, 1998, vol. 124, no. 1, pp. 233) Conflict analysis for prediction of fatal crash locations in mixed traffic streams. Tiwari, G; Mohan, D; Fazio, J;Accident Analysis & Prevention. Vol 30(2), Mar 1998, pp. 207-2154) Influence of Traffic Signal Timing on Red-Light Running and Potential Vehicle Conflicts at Urban Intersections.Retting, Richard A.; Greene, Michael A. Transportation Research Record, 1997, no.1595, pp.1 Washington, DC;National Academy Press

Owing to the lack of information provided by the database searches a closer look was madeat the LTH, Department of Technology and Society (TFT) website looking at the currentresearch section and their list of publications stretching back over the last 10 years. Theresearch section suggests that there is a great interest in the further development of TCT

through the use of a video-based technique for image processing of all kinds of safety-relatedevents, from accidents, via serious conflicts to safe interactions. It is hoped that this moreobjective method of data collection will allow the development of a broader concept for safetyassessment. In response to this an e-mail was sent to Åsa Svensson to ask for moreinformation concerning how far the video technique had progressed. To date no response hasbeen received, and it is possible to assume that there are still many serious and potentiallylimiting factors that restrict the use of video recording to the point where is not yet viable fortraffic safety research in general. What does appear important from the website is that there isa distinct move from serious conflicts to all forms of road-user interaction to establish whatmight be termed a safety profile for a specific intersection. This formed the basis for ÅsaSvensson’s doctorate thesis completed in 1998.

To date no further publications on the shape of the severity hierarchy, originally assumed to

be in the form of a three-sided pyramid by Christer Hydén, have been forthcoming. A look atthe publication lists also reveals only a modest number of articles relating directly to the useof the conflict technique. These are listed below:

1) Shbeeb, Lina: Development of a traffic conflicts technique for different environments : a comparativestudy of pedestrian conflicts in Sweden and Jordan.Doctoral thesis. Bulletin 188, 20002) Hydén, Christer; Almqvist, Sverker: Traffic safety assessments based on traffic conflicts. Field studyin Kingston, Jamaica, No. 7136, 19953) ICTCT. The use of traffic conflicts technique and related methods for the evaluation of newtechnologies in road traffic with respect to safety and sustainability. Proceedings of 5th workshop ofICTCT Helsinki & Tallinn, Oct 92, Bulletin 115, 19934) SVENSSON Å. 1991, Vidareutveckling och validering av den svenska konflikttekniken (Developmentand validation of the Swedish traffic conflicts technique). LTH, Lund


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One further option has been to use a newly developed radar technique for conflict detection.The system developed by Sensys Traffic was tested at a site in Borlänge but proved to beunsuitable in its present form for this type of data collection. The radar technique requires agreat deal of adaptation for conflict detection, and although the company has expressed aninterest in the further development of a safety detection device, it is unlikely that any work willtake place in the near future owing to the present economical climate at Sensys.

By far the most important, interesting, and relevant work with regard to the EMV project, hasbeen carried out by Prof Tarek Sayed at the University of British Columbia in Vancouver,Canada. In a paper entitled “Traffic Conflict Standards for Intersections” from 1999 (a copy isattached). Sayed describes the application of the traffic conflict technique to estimate trafficsafety at intersections based on data collected from 94 conflict surveys at both signalised andunsignalised intersections, and has developed standards that allow for a relative comparisonof conflict risk using an Intersection Conflict Index. Additionally, regression analyses havebeen used to develop predictive models that relate the number of conflicts to traffic volumeand accidents. Work by Sayed intended to enhance the usefulness of the traffic conflicttechnique for intersection safety evaluation. Also, a case study is provided in the paper. Thiswork is also supported by two other papers (Sayed & Zein, 1998; Sayed 1997). Sayed hasalso considered the use traffic conflicts using micro-simulation for assessing safety standards

and many of his findings have had a significant influence on the current work in the SINDIproject and the further development of the HUTSIM windows based simulation tool (Zein, S.,Sayed, T., Nepomuceno, J., and deLeur, P., 1995; Sayed, T., Brown, G., and Navin, F.,1994).

Alternative:“ Objective” Measures of Traffic Conflicts

As stated above, the subjective nature of the conflict technique require considerable judgement by the conflict observer and have been criticised because of the inter-observer

variability in grading the severity of the evasive action. Alternative objective measures aresaid to include a cardinal or time-proximity dimension in the severity scale. The most commonmeasure of this type is “Time-to-Collision” (TTC) defined as the time it takes for two vehiclesto collide if they continue on their present trajectory at the same speed. The original definitionwas coined by Hayward in 1972, although there was a lot of very similar research at variousresearch establishments at this time. When there is a collision course the TTC-valuebecomes finite and decreases with time, the critical measurement in estimating conflictseverity is therefore the minimum TTC during the conflict. Furthermore, it should be noted thatTTC distributions have been applied in several studies to identify traffic safety impacts(Fancher et.al.,1997; Van Arem & De Vos, 1997). Variations on the TTC concept includePost-Encroachment Times (situations where no collision course prevails but the timedifference over a common spatial point is below a given threshold e.g. 1.5 secs), andDeceleration-to-Safety Time (see e.g. Hydén, 1996, Topp, 1998).

A new and highly interesting development of the TTC measure has been suggested byMinderhood and Bovy (2000). The authors suggest two new safety indicators based on TTCthat are useful for comparative road traffic safety analyses. In contrast to the classical TTCvalues that are measured at a specific cross section, the new indicators use vehicletrajectories collected over a specific time period for a certain roadway segment in order tocalculate a general safety indicator value. These values can then be analyses to determinevehicle-specific indicator values and safety-critical probabilities. The first of these newindicators is the Time Exposed TTC which measures the length of time that all vehiclesinvolved in conflicts spend under a designated TTC minimum threshold during a specifiedtime period. The second indicator is the Time Integrated TTC which uses the integral of theTTC profile of drivers to express the level of safety over the specified time period. Togetherthese values can be used to derive average values per vehicle and the probability of safety

critical situations per time unit. This new approach provides valuable comparative safety dataand also provides a very useful measure to calibrating simulation models.


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Safety Indicators and Micro-simulation

It is evident that traffic safety analyses with microscopic traffic simulation have a number of

restrictions. Most importantly, driver behaviour in real road traffic is more diverse and lesspredictable than can be implemented within any model whatever the level of detail. Normalmicroscopic simulation models are developed for e.g. traffic-flow analyses, and require farless consideration to driver behaviour and error modelling than that essential for safetyanalyses. There is also little or no lateral vehicle movement. However, despite these andvarious other limitations it is believed that this type of simulation can give valuable insightsinto the relative safety impacts brought about by changes of traffic flow, various ITS devicesplaced inside the vehicle or on the roadway, different signalling strategies, and many otherdependent aspects. Measuring safety indicators such as TA, and TTC that emanate from themicroscopic simulation of vehicle dynamics and driver behaviour (including the probability oferrors) in the traffic environment is an essential part of the SINDI project.

References:Grayson, G.B. (1984). The Malmö study. A calibration of traffic conflict techniques. Institute for RoadSafety Research SWOV, Leidscendam

Hauer, E. (1978). Traffic conflict surveys: some study design considerations. TRRL SupplementaryReport 352. Transport and Road Research Laboratory, Crowthorne, Berkshire, UK

Hydén, C. (1987). The development of a method for traffic safety evaluation: The Swedish TrafficConflicts Technique. Bulletin 70. Institute för Trafikteknik, LTH,Lund

Hydén, C. (1996). Traffic Conflicts Technique: State-of-the-art. In: Topp H.H. (Ed.), (1998). Traffic SafetyWork with video-processing. University Kaiserslautern. Transportation Department, 1998, Green SeriesNo.43

Migletz, D.J., Glauz, W.D. & Bauer, K.M. (1985) Relationships between traffic conflicts and accidents.

Report No: FHWA/RD-84/042. US Department of Transportation, Federal Highway Administration.Perkins, S.R., & Harris, J.I. (1968). Traffic conflict characteristics: Accident potential at intersections.Highway Research Record. 225, pp45-143, Highway Research Board, Washington DC

Sayed, T. (1997) "Estimating the Safety of Unsignalized Intersections Using Traffic Conflicts" ThirdInternational Symposium on Intersections without Traffic Signals, Transportation Research Board,Oregon, July 21-23.

Sayed, T., and Zein, S. (1998). "Traffic Conflict Models and Standards for Signalized and UnsignalizedIntersections" , Proceedings of the CSCE 2nd Transportation Specialty Conference, June 10-13,Halifax.

Sayed, T., and Zein, S. (1999). "Traffic Conflict Standards for Intersections" , Transportation Planningand Technology , Vol. 22, pp. 309-323.

Sayed, T., Brown, G., and Navin, F., (1994) "Simulation of Traffic Conflicts at Unsignalized IntersectionsWith TSC-Sim", AccidentAnalysis and Prevention, Vol. 26(5), pp. 593-607. Spicer, B.A. (1973). Study of traffic conflicts at six intersections. TRRL Report LR551. Transport andRoad Research Laboratory, Crowthorne, Berkshire, UK

Svensson, Å. (1992). Vidareutveckling och validering av den svenska konflikttekniken (Developmentand validation of the Swedish traffic conflicts technique). LTH, Lund

Topp, H.H. (Ed.), (1998). Traffic Safety Work with video-processing. University Kaiserslautern.Transportation Department, 1998, Green Series No.43

Zein, S., Sayed, T., Nepomuceno, J., and deLeur, P., (1995) "Advancements in Traffic ConflictResearch: Standards and Simulation", 1995 Annual Conference, Transportation Association of Canada(TAC), Victoria, Canada,October 22-25.

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