Model Included variables Adjusted R2 Significance 1 2 3 4 5 Tracking divided attention test Tracking divided attention test RT Sternberg memory scanning test Tracking divided attention test RT Sternberg memory scanning test Tracking (hard condition) Tracking divided attention test RT Sternberg memory scanning test Tracking (hard condition) Error percent Sternberg memory scanning test Tracking divided attention test RT Sternberg memory scanning test Tracking (hard condition) Error percent Sternberg memory scanning test RT divided attention test R 0.470 0.531 0.562 R2 0.221 0.282 0.316 0.578 0.334 0.219 0.279 0.311 0.327 0.585 0.342 0.334 p=0.020 Psychopharmacology (2012) 220:293–301 Table 3 Results from the stepwise regression analysis to determine which test parameters predict changes in SDLP 299 p=0.0001 p=0.0001 p=0.0001 p=0.001 RT reaction time errors in the divided attention test did not significantly contribute to the predictive validity of the model. Discussion The objective of the presented analyses was to determine to what extent performance on the psychometric tests predicts on-the-road driving performance. The data show that our laboratory test, composed of a variety of skills and abilities related to driving has insufficient power to predict on-road driving performance, as expressed in SDLP. Although some skills such as tracking performance correlate well with road tracking (SDLP), the tests measuring tracking skills are not suitable to replace on-road testing. Tables 1 and 2 show that impairment found on the road (i.e., significant SDLP increment relative to placebo) is not consistently seen for parameters of the psychometric tests. Also, sometimes skills related to driving are unimpaired when tested in isolation, whereas the on-road driving assessment shows significant impairment. Taken together, the results of the analyses do not advocate replacing on-road driving assess- ments by psychometric tests measuring related skills and abilities. The studies that were used for the analyses included a limited number of psychometric tests. These tests were carefully selected, based on the models of driving perfor- mance described in the introduction of this paper, and the quality of the tests. It was aimed to include tests that had a clear rationale and measured psychological constructs that are relevant to driving. For example, the Sternberg memory scanning test is based on a sound theory of short-term memory and information processing theory, and a compre- hensive background literature is available on the test and its parameters (Sternberg 1966, 1975). One of the fundamental propositions of information processing theory is that the capacity of the cognitive systems is limited (Schneider and Shiffrin 1977; Shiffrin and Schneider 1977). Dual tasks, such as the divided attention test included in the present test battery, are used to investigate how subjects allocate resources among the tasks and how this affects perfor- mance. In the divided attention test, attention is divided among two components that are vital to driving a car. That is, tracking places demands on motor-related resources, whereas memory scanning places demands on working memory. It is therefore not surprising that its parameters, tested individual or in the divided attention test, correlate significant with SDLP, and do so better than parameters from tests that are less clearly related to driving. This comparison of psychometric test results with SDLP, measured on the road in normal traffic, confirms previous findings by Ramaekers (2003) that results from psychometric tests poorly predict SDLP. The current analysis also found that tracking tests showed the best correlation with SDLP. Combining the test parameters increases the predictive validity only by about 10%, relative to individual tests. In the past, Volkerts et al. (1992) compared performance on the on-the-road driving test with measurements in a driving simulator. The TS2 driving simulator test consisted of a number of subsequent curve-following maneuvers (road tracking), while simultaneously, subjects had to react to visual signs by button press. The number of correct maneuvers and reaction time were the two outcome measures. Treatments (lormetazepam 1 mg, oxazepam 50 mg, and placebo) were administered and subjects performed an on-road driving test and simulator driving test the morning following night 1 and night 2. On both test