Key Theory And Method For Dilemma-zone Avoidance-guiding System Design At Signalized Intersections

Dilemma zone (DZ) located at signalized intersections increases the occurrence oftraffic conflicts. At the onset of a yellow phase, vehicle being in the DZ can neither crossthe intersection before the onset of a red phase nor stop safely, resulting in running red orbraking abruptly, and therefore conflicts such as rear-end and right-angle are likely tooccur. In essence, there are two leading reasons for vehicles being caught in the DZ. First,the boundary of the DZ is influenced by various factors, including driver characteristics,vehicle dynamics, intersection parameters, and signal control maneuver, which makes theDZ exist objectively and dynamically; second, driver may make a wrong choice ordecision when approaching the intersection and cannot determine whether the subsequentdriving behavior will lead the vehicle into a DZ. To cope with the DZ issue, severalmethods have been proposed, such as extending the yellow phase, installing warningsigns, deploying green extension system and applying onboard warning system. Althoughthe number of vehicles in the DZ can be reduced significantly, the existing methods aredeficient, because they do not fully consider the dynamic nature of the DZ and itscontributing factors (e.g., signal control maneuver); thus, vehicles still have the possibilityof being caught in the DZ at the onset of a yellow phase.If the appropriate guidance can be provided for vehicles approaching the intersectionbefore the onset of a yellow phase, and the subsequent driving behavior can bedetermined, then the risk of being caught in the DZ will be further reduced. Therefore,taking into account the primary reasons for encountering a DZ, this thesis focuses on howto provide vehicles with detailed and feasible guidance, and then an advance-guidingbased DZ-avoidance system is built. As the key points, the DZ-guiding algorithm and themethod for determining the activation time of the proposed system are given a specialattention. The main contributions of the thesis are as follows:(1) The issues related to DZ and DZ-avoidance methods are analyzed andsummarized. From the prospective of physical location and stopping probability, thedefinitions of the two types of DZ, as well as the methods for determining their boundaries are presented, respectively; based on the critical crossing distance and thecritical stopping distance, driving behavior at signalized intersections and its influencingfactors are analyzed; in addition, the developing trend of the DZ-avoidance methods ispointed out, and the deficiencies of the existing methods are summarized.(2) An advance-guiding based DZ-avoidance system is built. Along with the systemdesign, vehicle status and space-time status needed for the system are given first; then,according to the presence of the leader vehicle and its driving behavior, two disparateguiding methods of the system are determined, i.e., calculating guiding strategies orgenerating warning information; besides, the process of the guiding system undercooperative vehicle-infrastructure environment is presented. Since accurate boundary ofthe DZ is the prerequisite for providing reliable guidance, the methods for computing theboundary with/without an all-red phase are proposed, respectively; with the computedresult, the initial vehicle DZ state is analyzed, which can provide support for theformulation of a detailed guiding strategy. Specifically, other relevant issues involved indesigning the guiding system are described as well, including the restriction of theacceleration and deceleration rate, the choice of the car-following model, and theapplication of the longitudinal control system, so that the proposed DZ avoidance-guidingsystem can be in line with the actual driving environment.(3) A DZ-guiding algorithm is proposed to calculate the guiding strategies. Both theapplication scope and design idea are given, and the flow of the proposed algorithm whenvehicles to be located in the DZ, stopping zone and crossing zone is described,respectively; in particular, under the premise of satisfying driver’s comfort, car-followingsafety and local speed limit, a minimum acceleration rate required for preventing vehiclesfrom being caught in the DZ is computed in terms of different signal control maneuvers;For the case where a vehicle is able to cross at current speed or needs to brake abruptly, anacceleration rate or a deceleration rate required for the vehicle to maintain a safe distancefrom its leader is computed.(4) Given the fact that the DZ avoidance-guiding system will be activated at somepoint before the onset of a yellow phase, and there may be drawbacks by setting theactivation time artificially, a method for determining the activation time is proposed.Assuming the DZ exists, the impacts of the activation time on the vehicle that perform anaccelerating/decelerating strategy for DZ-avoidance are analyzed synthetically. Inaddition, based on the signal control maneuver with/without an all-red phase, theactivation time required for vehicles that would have been caught in the DZ to cross/stop with a guiding strategy, is computed through the established mathematical relationshipbetween activation time and approaching speed.The main innovative points of the thesis are as follows:(1) An idea for addressing the DZ issue is proposed from the perspective ofproviding the vehicles with detailed guidance in advance. An advance-guiding basedDZ-avoidance system is built, which can cope with the dynamic nature of the DZ andprevent vehicles from being caught in Type I or Type II DZ.(2) Based on the restrictions of the yellow and all-red phase, for the case where theall-red phase exists, the boundaries of the DZ for vehicles with different approachingspeed are specified.(3) Car-following theory is taken into account when designing the DZ-avoidancemethod, which gives sufficient consideration to the impacts of car-following safety onguiding strategies that used for DZ-avoidance. The methods for computing theaccelerating/decelerating rates which could prevent vehicles from encountering the DZare presented with regard to three cases, including vehicles that would have been caughtin the DZ, vehicles that are able to cross at current speed, and vehicles that need to brakeabruptly.(4) Based on the signal control maneuver with/without an all-red phase, theactivation time required for vehicles that would have been caught in the DZ to cross/stopwith a guiding strategy, is computed through the established mathematical relationshipbetween activation time and approaching speed.This paper focused on how to provide vehicles with detailed and feasible guidance,so that the risk of encountering a DZ can be mitigated. The proposed methods not onlyenrich the theory of the DZ issue, but also suggest a new idea for future research onDZ-avoidance.