ON-ROAD TESTING

Naturalistic Studies (Driving In Traffic) continued...

On-road measures based on a modification of the California Driver Performance Evaluation (DPE) were used in research reported by Janke (2001) and Janke and Hersch (1997). The DPE measures these constructs: visual search, speed control, and directional control. It is designed to detect a fixed number of possible errors, and features objective criteria for scoring specific (structured) maneuvers at specific locations. Examples of structured maneuver errors include “inadequate traffic check,” “poor lane position,” and “turns too wide (or too short).” The Modified Driver Performance Evaluation (MDPE) differs from the DPE in that it omits freeway driving, while adding a destination-finding task that requires a subject to safely return to the field office after being directed to drive a short distance past the office. The destination driving task was incorporated to test for possible cognitive impairment. Test times range from 30 to 45 minutes. The MDPE includes these maneuvers:

• 4 left turns and 4 right turns (mixed difficulty levels; 2 left turns and 2 right turns with multiple lanes requiring correct lane choice on approach and finish; 1 left and 1 right turn at signal controlled intersections; 2 additional turns, preferably at stop controlled intersections but may be uncontrolled with limit lines, crosswalks, turn lanes)
• 8 through intersections (2 controlled by a light [red, yellow, and green]; 2 controlled by a stop sign; 2 through/straight ahead intersections not involving stops; 2 additional intersections preferably controlled by traffic lights)
• Merging at a lane drop
• Curve negotiation (preferably a left curve; lanes should be marked, must require driver to adjust speed)
• 3 lane changes (1 left, 1 right, 1 located anywhere on the route, preferably at higher speeds)
• Minimum of 3 blocks of driving in a business area with moderate traffic density
• Minimum of 3 blocks of driving in a residential area preferably with narrow streets
• Performing parking lot maneuvers, backing, and a street park

A subset of errors defined as critical driving errors was listed in a separate section of the DMV score sheet. These are serious errors; under normal testing circumstances (i.e., other than a research situation), a driver’s test would immediately be terminated. Such errors include: examiner intervention required (to avoid crash); driver strikes object; drives up/over curb or sidewalk; drives in oncoming traffic lane; disobeys sign/signal; dangerous maneuver; inappropriate reaction to school bus; inappropriate reaction to emergency vehicle; inappropriate speed; inappropriate auxiliary equipment use; turn from improper lane.

A subset of critical errors was also defined as hazardous errors, with the belief by the CA DMV that these errors are the strongest indicators of driving impairment. The hazardous error subset included “dangerous maneuver” and “examiner intervention required.”

A weighted error score served as the primary criterion (dependent) variable for the driving performance analyses. This score was calculated by adding the total number of errors (regardless of severity) to twice the sum of critical and hazardous errors. Since hazardous errors were a subset of critical errors, and critical errors were a subset of total errors, this scheme weighted hazardous errors by a factor of five and other critical errors by a factor of three.

Confusion (concentration) errors were also recorded, when a subject was unable to proceed to field office at end of test, or drove past the street on which the field office was located and did not recognize their error.