There is a seemingly perennial debate in the literature about the relative merits of
using a secondary task as a measure of spare attentional capacity. One of the main
drawbacks is that it could adversely affect the primary task, or other measures of
mental workload. The present experiment therefore addressed an important
methodological issue for the dual-task experimental approach – that of secondary task
interference. The current experiment recorded data in both single- and dual-task
scenarios to ascertain the level of secondary task interference in the Southampton
Driving Simulator. The results indicated that a spatial secondary task did not have a
detrimental effect on driving performance, although it consistently inflated subjective
mental workload ratings. However, the latter effect was so consistent across all
conditions that it was not considered to pose a problem. General issues of
experimental design, as well as wider implications of the findings for multiple
resources theory, are discusse
