12.2 Task Analysis of Driver Merging Behavior at Freeway Entrance Ramps

Merging onto a freeway is a dynamic, challenging task fraught with crash risk. Motorists on the mainline and those who are attempting to merge onto the mainline identify gaps and modify their speeds accordingly. Table 12.1 decomposes merging into five tasks.
Drivers tend to merge more aggressively on ramps that have poor geometry. If geometrics are good and traffic volumes are high, drivers tend to use most of the acceleration lane to merge onto the mainline.

Table 12.1 Elements of Merging
Task	Distance/Time Required	Driver/Roadway Factors
Initial Steering Component Motorist transitions from entrance ramp to speed-change lane.	≈ 1 second	—
Acceleration Motorist speeds up so to get an unobstructed view of mainline freeway traffic.	≈ 6.6 ft/s² (85th percentile maximum comfortable acceleration)	Influenced by controlling ramp curvature
Gap Search Once they see the ramp nose downstream, the motorist looks for a gap in mainline traffic to merge into.	≈ 0.25 – 0.5 seconds	Difficult to judge the speed and position of vehicles approaching on the mainline Variability in gap acceptance among drivers In congested conditions, zip merging occurs at the end of the ramp
Merge Steering Motorist transitions from the speed-change lane to the mainline.	85% of vehicles merge within ≈ 1,230 ft Merging behaviors do not improve when speed-change lanes are > 1,394 ft	On tapered ramps, motorists use a longer portion of the ramp to complete a merge. They can also merge more aggressively.
Abort A motorist that cannot find a gap in the mainline and then decelerates before the speed-change lane ends.	Varies based on the angular velocity of the approaching ramp end.	—

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