The present study examined a model of the role of cognitive functions related to attention and decision making in merging manoeuvres during simulated highway driving. To test this model, we screened participants who scored high or low on two sets of tasks that were assumed to represent attentional function and decisiveness. We then conducted a driving simulation experiment in which attentional function and decisiveness were between-subject factors and task difficulty was varied as a within-subject factor. The results indicated that the attentional function was the primarily determinant of the swiftness and success of merging. The role of decisiveness was confined primarily to the activation of the turn signal. Thus, the present experiment suggests that attention is the cognitive function that determines performance during merging behaviour in the setting under examination. Exploratory multivariate analyses confirmed the measureable behaviour obtained from the simulation. The present results provided information about the cognitive functions for merging behaviour that may be useful to the automobile industry.