6. LaneChangingModels

SimMobility adopts the integrated driving behavior modeling framework based on the concepts of short-term goal and short-term plan. Driving behavior consists of three main elements: the short-term goal, the short-term plan, and the driver's actions. The short-term goal is defined by the driver's target lane. The driver constructs a short-term plan, which is defined by the target gap in the target lane that driver wishes to use in order to accomplish his goal (i.e., move to the target lane). The actions are the 2-dimensional movements of the vehicle (accelerations and lane changes - LC) that the driver performs in order to execute the short-term plan. In the case that the driver does not intend to change lanes, he may be viewed as executing a passive plan to follow the leader or attain the desired speed. The model hypothesizes four levels of decision-making: target lane, gap acceptance, target gap and acceleration. This decision process is latent. The short-term goal (target lane) and short-term plan (target gap) are both not observable. Only the driver's actions are observed.

At the highest level, the driver chooses a target lane. The target lane is the lane the driver perceives as best to be in. This is the choice of short-term driving goal. The current branch corresponds to a situation in which the driver decides not to pursue a LC. In this situation car-following or free-flow behaviors are active based on the relations with the vehicle in front. In the case that either the right lane or the left lane are chosen, the driver evaluates the adjacent gap in the target lane and decides whether this gap can be used to execute the LC or not. If the gap is accepted, the LC is immediately executed and the short-term goal is accomplished. The acceleration that the driver applies in this case is determined by car-following or free-flow behavior with respect to the leader int he new lane.

Structure of the driving behavior model

If the available gap is rejected , the driver evaluates available gaps in the target lane and chooses the one that would be used to perform the desired LC. This choice defines the short-term plan that the driver would follow. The target gap choice set may include alternatives that do not correspond to specific gaps in traffic. For example, the driver may decide to look for gaps in traffic upstream of his current location (i.e. gaps between vehicles that are currently behind him) without committing to a specific gap. The acceleration the driver applies is determined such that to facilitate the execution of the short-term plan (i.e. the driver tries to position the vehicle in a way that will increase the probability that the target gap will be acceptable). The acceleration may also be constrained by car following considerations, since the LC is not immediate. It is worth noting that LCs and accelerations are represented differently in the model. LCs are modeled as discrete events occurring at specific points in time. Therefore, a lane changing observation (or absence of one) is assumed to take place during the time interval being modeled. The assumption, similar to Markov models, is that only one LC may be executed during each time interval. This assumption is reasonable since in reality LCs are not performed instantaneously and given that the length of time intervals is short relative to the execution time of LCs. The reader is referred to the following documents for further technical details:

• Toledo, T. (2003). Integrated Driving Behavior Modeling. Phd. Thesis. Massachusetts Institute of Technology, Cambridge, USA, 2003.

• Choudhury, C. F. (2007). Modeling Driving Decisions with Latent Plans. Environmental Engineering. Massachusetts Institute of Technology, Cambridge, USA, 1999.

The lane choice by the subject vehicle is specified in the file sim_mob::MITSIM_LC_Model