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1、Chapter 3Traffic Conflict Studies A.Traffic Safety Data Analysis Methods 1.Purposes of Safety Analysis-To find an area with potential safety problems-To study the safety performance of a particular safety countermeasure-To predict or estimate safety impacts of factors related to roadway,traffic,and
2、control conditions 2.Basic Safety Data Analysis Methods-Crash data analysis-Crash modeling-Traffic conflict analysis 3.Basic Problems with Crash Analysis-The first problem is with the reliability of police reported crash records because the number of crashes tends to be under-reported.-Crash databas
3、es are subject to human error during the input of data.Therefore,code errors,reporting errors and even interpretation errors may appear.-The waiting time for obtaining an adequate sample size may be too long.-Other concerns are the randomness of crashes and the human factors associated with them.B.C
4、oncept of Traffic Conflicts 1.Interaction between Vehicles-Conflict points:A conflict point is defined as the point at which two traffic movements intersect each other.2.Definition of Traffic Conflicts(several definitions)-A traffic conflict was defined as an evasive action taken by a driver in orde
5、r to avoid a collision.-A conflict is an observable situation in which two or more road users approach each other in space and time to such an extent that there is a risk of collision if their movements remain unchanged.-A traffic conflict is defined as an event involving two or more road users,in w
6、hich the action of one user causes the other to make an evasive maneuver to avoid a collision.(Show video CD)3.Benefits to Use Traffic Conflicts-Data can be collected within a short period of time so that an engineer does not have to wait for the occurrence of several crashes to improve the conditio
7、ns of a site.-The effectiveness of a treatment can be evaluated within a short period and if this fails to correct the problem then the countermeasure can be changed again in a very short time.-Traffic conflicts include human factors because the behavior of drivers can directly be observed in the fi
8、eld.-Traffic conflict studies can be used with or without crash data since each type of conflict is associated with a particular type of crash.-Traffic conflict data provides information about traffic volumes,routine conflicts,moderate conflicts,erratic maneuvers,severe conflicts or near-miss crashe
9、s,and other minor crashes,while crash data can only give information on property damage and injury severity C.Methods of Identifying Traffic Conflicts 1.Number of Observations Required-Manual of Transportation Engineering Studies N=number of vehicles to be countedp=expected proportion of vehicles ob
10、served that are involved in a conflictz=statistic that is based on the level of significance desiredD =permitted level of absolute error of sample size-Example,if one wants to find the sample size,with a level of significance of 5 percent,and with an absolute error of 5 percent,the number of vehicle
11、s would be n=0.50(1-0.50)(1.96/.05)2=384 approach vehicles 2.Identification of Traffic Conflicts-Evasive maneuvers such as applying brakes,swerving,or noticeably decelerating in order to avoid a collision can be considered as conflicts.-Brake applications have been usually used as indicators of the
12、occurrence of a conflict.-Swerving is also used as an indicator of the occurrence of a conflict,although it may not sometimes be clear whether swerving was because of a conflict or not.-Using perception of deceleration of a vehicle is useful for detecting conflicts when there are no brake light indi
13、cations because of a mechanical failure.-Observers are the most important element when conducting a traffic conflict study because their reliability has a serious impact on the validity of the data.Therefore,training and educating observers are the most important factors considered in the initial st
14、ages.D.Analysis of Traffic Conflicts 1.Conflict Severity-Based on conflict type,such as:right-turn conflict is more severe than rear-end conflict.-Based on field observation:(1)objective method and(2)subjective method2.Conflict Severity Evaluated by Objective Method-Objective methods rely on physica
15、l properties such as time,distance,and speed.-Time to Collision(TTC)is used to evaluate conflict severity.TTC is defined as the time required for two vehicles to collide if they continue at their present speeds on the same paths.3.Conflict Severity Evaluated by Subjective Method-Subjective measures
16、rely on human observers to record the perceived risk at the moment in which the conflict occurred.-Risk Of Collision(ROC)is used to evaluate the severity of conflicts,ROC is based on the severity of the risk perceived while collecting data in the field.4.Severity Score ValuesTTC and ROC ScoresTTC(se
17、conds)ROC11.51 2.00Low Risk21.00 1.50Medium Risk30.00 0.99High Risk5.Conflict Risk Index RIj=total risk index for intersection jRIij=risk index for conflict type i at intersection jKi=relative weight for conflict type iWi=weighting factor for conflict type i(the weighting factor is based on a subjec
18、tive scale,which ranged from 1 to 3,1 for the least severe conflict,and 3 for the most severe conflict-could use the severity score values.)(I.V.)ij=indicator value for conflict type i at intersection j(the indicator value corresponds to the number of conflicts per thousand entering vehicles on the
19、site conflict rate.)n=number of conflict types 6.Conflict Rate CR1 =conflict rate 1CR2 =conflict rate 2V1and V2=interacting traffic volumesRate TypeDefinitionConflicts per HourConflict Per Thousand Involved VehiclesE.Application of Conflict Analysis(A Real Research Project Sponsored by FDOT)1.Basic
20、Technical Issues-Direct left-turn(DLT)movements create problems.-Right-turn followed by U-turn(RTUT)could be an alternative.-There were no field data to prove the benefit of RTUT.2.Research Objective-To quantify safety impact of RTUT-Identify threshold values for determining when it is appropriate t
21、o implement the RTUT technique.3.Main Measure for Traffic Safety Analysis-Traffic conflict study was used.-Why traffic conflict study:-Data collected in short period of time.-Conflicts include human factors.-Conflicts provide more information.4.Traffic Conflict Data Collection-Used video cameras to
22、record traffic movements.-Video tapes were reduced to obtain traffic movement data and conflict data.-Good weather,normal traffic conditions,and dry pavement-Data collection sites-8 sites(arterial with 6 or more lanes)-Traffic volume on driveway should be relatively high.-Effects of upstream and dow
23、nstream signals should be minimum.5.Conflict Types Studied-Type C1.Right turn out of driveway-Type C2.Slow-vehicle same direction-Type C3.Lane change conflict-Type C4.U-turn conflict-Type C5.DLT,conflict from right-Type C6.DLT and left-turn in from right-Type C7.DLT and left-turn in from left-Type C
24、8.DLT conflict from left-Type C2U-T.Slow U-turn vehicle,same direction conflict6.Conflict Data Analysis-Descriptive analysis-Data analysis-Conflict rates:conflicts per hour and conflict per thousand involved vehicles-Conflict severity-“Before and After”study EndHomework No.2Potential Collision Situa
25、tionConflictNo ConflictEvasive ActionNo Evasive ActionCollisionNo CollisionSide Street/DrivewayMajor RoadConflict Point16 Major Conflict Points of Direct Left Turns MovementsSide Street/DrivewayMajor RoadConflict Point4 Major Conflict Points of Right Turn Followed by U-turn MovementsDescription of S
26、ites SelectedData Collection FormConflict type C1Conflict type C2Conflict type C3Conflict type C4Conflict type C5Conflict type C6Conflict type C7Conflict type C8Conflict type C2U-TDescriptive AnalysisDescriptive AnalysisDaily average number of conflicts for RTUT movements by conflict typeDescriptive
27、 AnalysisDaily average number of conflicts for DLT movements by conflict type.Descriptive AnalysisAverage number of conflicts per hour of observation for RTUT movementsAverage number of conflicts per hour of observation for DLT movementsComparison of Average Number of conflicts per hour between RTUT
28、 and DLT movements,by time period.Average severity of conflicts based on ROC ScoreAverage severity of conflicts based on ROC and TTC ScoresANOVA Results for ROCANOVA Results for ROC and TTCSite for“Before and After”StudyAverage number of conflicts per hour of observation for DLT movements “before”im
29、provement.Average number of conflicts per hour of observation for RTUT movements“before”improvement.Average number of conflicts per hour of observation for RTUT movements“after”improvement.Homework No.21.Each student should search internet to find a reference paper related to traffic conflict study.2.Each student should review the video to:(1)Specify conflict types(2)Count conflicts for each conflict type(3)Present study results in certain form