Classification of Lane change

The classification of lane changes is done based on the execution of the lane change and accordingly two type of lane changes exists.

Mandatory Lane Change (MLC):

Mandatory lane change (MLC) occurs when a driver must change lane to follow a specified path Suppose if a driver wants to make a right turn at the next intersection the he changes to the right most lane which is referred as Mandatory Lane change.

Discretionary Lane Change (DLC):

Discretionary lane change (DLC) occurs when a driver changes to a lane perceived to offer better traffic conditions, he attempts to achieve desired speed, avoid following trucks, avoid merging traffic, etc. Suppose if a driver perceives better driving conditions in the adjacent lane then he makes a Discretionary Lane change.

**Figure 1:** Discretionary Lane change model
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MLC and DLC Model

The lane changing model structure is shown in Figure 1. The MLC branch in the top level corresponds to the case when a driver decides to respond to the MLC condition. Explanatory variables that affect such decision include remaining distance to the point at which lane change must be completed, the number of lanes to cross to reach a lane connected to the next link, delay (time elapsed since the MLC conditions apply), and whether the subject vehicle is a heavy vehicle (bus, truck, etc..,). Drivers are likely to respond to the MLC situations earlier if it involves crossing several lanes. A longer delay makes a driver more anxious and increases the likelihood of responding to the MLC situations. And finally, due to lower maneuverability and larger gap length requirement of heavy vehicles as compared to their non heavy counterparts, they have a higher likelihood of responding to the MLC conditions.

Decision making

The MLC branch corresponds to the case where either a driver does not respond to an MLC condition, or that MLC conditions do not apply. A driver then decides whether to perform a discretionary lane change (DLC). This comprises of two decisions: whether the driving conditions are satisfactory, and if not satisfactory, whether any other lane is better than the current lane. The term driving conditions satisfactory implies that the driver is satisfied with the driving conditions of the current lane. Important factors affecting the decision whether the driving conditions are satisfactory include the speed of the driver compared to its desired speed, presence of heavy vehicles in front and behind the subject, if an adjacent on ramp merges with the current lane, whether the subject is tailgated etc.

If the driving conditions are not satisfactory, the driver compares the driving conditions of the current lane with those of the adjacent lanes. Important factors affecting this decision include the difference between the speed of traffic in different lanes and the driver's desired speed, the density of traffic in different lanes, the relative speed with respect to the lag vehicle in the target lane, the presence of heavy vehicles in different lanes ahead of the subject etc. In addition, when a driver considers DLC although a mandatory lane change is required but the driver is not responding to the MLC conditions, changing lanes opposite to the direction as required by the MLC conditions may be less desirable.

If a driver decides not to perform a discretionary lane change (i.e., either the driving conditions are satisfactory, or, although the driving conditions are not satisfactory, the current is the lane with the best driving conditions) the driver continues in the current lane. Otherwise, the driver selects a lane from the available alternatives and assesses the adjacent gap in the target lane.. When trying to perform a DLC, factors that affect drivers' gap acceptance behavior include the gap length, speed of the subject, speed of the vehicles ahead of and behind the subject in the target lane, and the type of the subject vehicle (heavy vehicle or not). For instance, a larger gap is required for merging at a higher travel speed. A heavy vehicle would require a larger gap length compared to a car due to lower maneuverability and the length of the heavy vehicle.