THE EFFECT OF SPEED AND SPEED CHANGE ON DRIVERS' SPEED JUDGEMENT

PREVIOUS RESEARCH INTO SPEED ESTIMATION IN A DRIVING SITUATION HAS INDICATED THAT THE DRIVERS'S ACCURACY OF JUDGEMENT WAS NOT ONLY AFFECTED BY VEHICLE SPEED BUT MIGHT ALSO BE FUNCTION OF BOTH DEGREE AND DIRECTION OF SPEED CHANGE. THE REALITY OF THIS RELATIONSHIP WAS INVESTIGATED BY THE USE OF A TECHNIQUE IN WHICH MEMBERS OF THE GENERAL PUBLIC, ACTING AS SUBJECTS, WERE ASKED TO DRIVE A VEHICLE AT A NUMBER OF TEST SPEEDS RELATED TO A PREVIOUS SPEED BY A GIVEN RATIO. FIVE DIFFERENT RATIOS WERE USED. THE OBJECTIVE SPEED RATIO PRODUCED WAS THEN EXPRESSED AS A FUNCTION OF BOTH SPEED AND SUBJECTIVE OR REQUIRED SPEED RATIO. EXPERIMENTAL RESULTS WERE PLOTTED AGAINST A FAMILY OF PREDICTED CURVES DERIVED FROM AN EARLIER EXPERIMENT. THE DATA WAS ALSO EXPRESSED IN TERMS OF PERCENTAGE ERROR OF SPEED CHANGE RESULTS FROM DECELERATION, CHANGES OF SPEED ARE SHOWN TO AFFECT ACCURACY OF JUDGEMENT MORE THAN SPEED ITSELF. FOLLOWING ACCELERATION, ACCURACY IS AFFECTED BY BOTH SPEED AND SPEED RATIO. THE RELEVANCE OF THESE RESULTS TO THE ROAD SITUATION IS DISCUSSED. /AUTHOR/