Measuring the Spare ‘Mental Capacity ’of Car Drivers by a Subsidiary Task

It is impossible to determine the degree to which a driver is absorbed in his driving by measuring his overt responses directly. However, relatively small changes in his spare ‘ mental capacity ’ can be detected by scoring his performance on a subsidiary task, which has no adverse effect upon driving. This technique is sufficiently sensitive to reveal the higher level of concentration required in a shopping area as compared with that required in a residential area