Naturalistic Studies (Driving In Traffic) continued...
In the Di Stefano and Macdonald (2003) study, the most common errors (across all 533 tests) made during intersection negotiation tasks were: failure to check mirrors (69% of tests), failure to use turn signals (49% of tests), poor gap selection/judgment (43% of tests), poor positioning on the road when turning (39% of tests), failure to obey the sign or signal (30% of tests), and poor approach (unsafe speed before an intersection/rough deceleration) on 14 percent of tests. The most common errors related to lane changing included: failure to turn the head to check back over the shoulder (62% of tests), failure to use turn signals (31% of tests), failure to check mirrors (26% of tests), and poor gap selection (10% of tests). The most common errors related to low-speed maneuvers included: failure to turn the head to check back over the shoulder (45 % of tests), failure to check mirrors (13% of tests), and failure to use turn signals (12% of tests). The fail rate was 49 percent (261 of 533 tests).
On all but 9 of the tests where there was a failure, there was at least one LTO inter-vention; for tests with an intervention, the mean number per test was 3.6 with a maximum of 12. The most frequent problems causing LTO intervention involved errors associated with intersection negotiation, failing to yield or poor gap selection, failing to maintain the vehicle in the appropriate position on the road, inappropriate speed (either too fast or too slow), and problems with low-speed maneuvers. Although test outcome was almost always determined by LTO intervention, the variables most strongly associated with failing were scores relating to intersection negotiation, maintenance of position and speed, and safety margin. Not surprisingly, these are the types of performances associated with LTO intervention. The authors compared the errors leading to LTO intervention in this study to the “hazardous errors” reported by other researchers.
Di Stefano and Macdonald (2003) also looked at the influence of medical condition on road test fail rate. The most commonly reported medical conditions were cardiac (e.g., hypertension), endocrine (e.g., diabetes), musculoskeletal (e.g., soft tissue disorders), visual (e.g., only one functioning eye), arthritis (e.g., osteoarthritis), and mental/behavioral (e.g., affective disorders). There was no relationship between number of medical conditions and test outcome, nor was there a significant relationship between category of medical condition (primarily physical, primarily cognitive, or mixed) and test outcome (pass or fail). There was, however, an apparent effect of physical impairment on car control, where arthritis emerged as the condition with the highest fail rate (61% of the subjects with arthritis failed).
There was a significant relationship between age and test outcome in this research. Fail rate increased sharply from 0 percent for subjects under age 54, to 38 percent for subjects 75 to 79, to 71 percent for subjects 85 and older. The highest correlations between performance scores and age were for intersection negotiation, followed either by position and speed (with test outcome) or lane changing (with age). These were followed by low-speed maneuvers, car control, and safety margin.
Di Stefano and Macdonald (2003) concluded that the set of performance scores developed in the study provide a generally valid indicator of the unsafe behaviors most typical of older drivers. This is supported by the relationship of the performance scores and LTO interventions, and by the fact that the addition of age to the regression model after entry of the performance scores explained very little additional variance.