北交大-交通安全3.ppt

1、Chapter 3,Traffic Conflict Studies,A. Traffic Safety Data Analysis Methods,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, traff

2、ic, and control conditions 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

3、. - Crash databases 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

4、associated with them.,B. Concept of Traffic Conflicts,Interaction between Vehicles - Conflict points: A conflict point is defined as the point at which two traffic movements intersect each other. Definition of Traffic Conflicts (several definitions) - A traffic conflict was defined as an evasive act

5、ion taken by a driver in order 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 invo

6、lving two or more road users, in which 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

7、several crashes to improve the conditions 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 o

8、f drivers can directly be observed in the field . - 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, er

9、ratic maneuvers, severe conflicts or near-miss crashes, and other minor crashes, while crash data can only give information on property damage and injury severity,C. Methods of Identifying Traffic Conflicts,Number of Observations Required - Manual of Transportation Engineering Studies N = number of

10、vehicles to be counted p = expected proportion of vehicles observed that are involved in a conflict z = statistic that is based on the level of significance desired D = permitted level of absolute error of sample size - Example, if one wants to find the sample size, with a level of significance of 5

11、 percent, and with an absolute error of 5 percent, the number of vehicles 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 conside

12、red as conflicts. - Brake applications have been usually used as indicators of the 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 decele

13、ration of a vehicle is useful for detecting conflicts when there are no brake light indications 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,

14、training and educating observers are the most important factors considered in the initial stages.,D. Analysis of Traffic Conflicts,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

15、) subjective method Conflict Severity Evaluated by Objective Method - Objective methods rely on physical 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

16、their present speeds on the same paths.,3. Conflict Severity Evaluated by Subjective Method - Subjective measures 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 o

17、n the severity of the risk perceived while collecting data in the field. Severity Score Values,Conflict Risk Index RIj = total risk index for intersection j RIij = risk index for conflict type i at intersection j Ki = relative weight for conflict type i Wi = weighting factor for conflict type i (the

18、 weighting factor is based on a subjective 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 c

19、onflicts per thousand entering vehicles on the site conflict rate.) n = number of conflict types,6.Conflict Rate CR1 = conflict rate 1 CR2 = conflict rate 2 V1and V2 = interacting traffic volumes,E. Application of Conflict Analysis (A Real Research Project Sponsored by FDOT) ,Basic Technical Issues

20、- 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 to

21、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

22、 cameras to 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. -

23、Effects of upstream and downstream 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 righ

24、t - Type C7. DLT and left-turn in from left - Type C8. DLT conflict from left - Type C2U-T. Slow U-turn vehicle, same direction conflict 6.Conflict Data Analysis - Descriptive analysis - Data analysis - Conflict rates: conflicts per hour and conflict per thousand involved vehicles - Conflict severit

25、y - “Before and After” study,End,Homework No. 2,16 Major Conflict Points of Direct Left Turns Movements,4 Major Conflict Points of Right Turn Followed by U-turn Movements,Description of Sites Selected,Data Collection Form,Conflict type C1,Conflict type C2,Conflict type C3,Conflict type C4,Conflict t

26、ype C5,Conflict type C6,Conflict type C7,Conflict type C8,Conflict type C2U-T,Descriptive Analysis,Descriptive Analysis,Daily average number of conflicts for RTUT movements by conflict type,Descriptive Analysis,Daily average number of conflicts for DLT movements by conflict type.,Descriptive Analysis,Average number of conflicts per hour of observation for RTUT movements,Average number of conflicts