Once a vehicle has decided to change lanes, it examines the lead and
lag gaps, 
and 
respectively in Figure 3.8, in
the target lane to determine whether the desired change can be
executed. If both the lead and lag gaps are acceptable, the desired
lane change is executed instantaneously.
Figure 3.8: Lead and lag gaps for lane
changing
The minimum acceptable gaps take into account the speed of the subject
vehicle ( 
), speed of the lead
and lag vehicles ( 
and 
), and the type of the lane change (i.e., discretionary or
mandatory). For discretionary lane changes, the critical gap is given
by:
where the subscript a indicates the lead vehicle, b the lag vehicle, and n the subject vehicle (see Figure 3.8).
For mandatory lane changes the critical gap is also a function of the distance of the vehicle from the downstream node (or incidents and lane drops). In other words, it is assumed that drivers tend to accept smaller gaps as they get closer to the last location where the lane change has to take place:
where:
, 
A more sophisticated lane change gap acceptance model has been proposed by [Ahmed et al.(1996)] and parameters are being estimated using field data. This model will eventually be used in MITSIM.
