A RATIONAL APPROACH TO PRACTICAL PROBLEMS OF SPEED ENFORCEMENT REQUIRES AN UNDERSTANDING OF THE MANNER IN WHICH ENFORCEMENT AFFECTS SPEED CHOICE. IN THIS REPORT, FOUR EXPERIMENTS ARE DESCRIBED. EACH EXPERIMENT CONSISTED OF MEASURING SPEEDS OF VEHICLES BEFORE, DURING AND AFTER ENFORCEMENT TOOK PLACE, ACCOMPANIED BY A CONTROL SECTION TO WHICH NO SPEED ENFORCEMENT WAS APPLIED. THE FOUR EXPERIMENTS DIFFER IN THE NUMBER OF DAYS OF ENFORCEMENT. DURING EXPERIMENTS 1 AND 2 CONSIDERABLE ATTRITION OF DATA OCCURRED. THEREFORE, CONCLUSIONS ARE BASED MAINLY ON DATA OBTAINED DURING EXPERIMENTS 3 AND 4. THE DATA BASE CONTAINS SOME 116,000 SPEED OBSERVATIONS. FOR SOME EXPERIMENTS, THE LICENSE PLATE RECORD OF VEHICLES WAS CODED. THIS ALLOWS THE TRACING OF THE SAME VEHICLE DAY AFTER DAY. ANALYSIS LEADS TO SEVERAL CONCLUSIONS. WHEN ENFORCEMENT IS IN PLACE, THE AVERAGE SPEED OF THE TRAFFIC STREAM IS REDUCED AT THE SITE OF ENFORCEMENT, UPSTREAM AND DOWNSTREAM OF IT. AT THE SITE OF SPEED LIMIT ENFORCEMENT, THE AVERAGE SPEED OF THE TRAFFIC STREAM IS AROUND THE POSTED SPEED LIMIT. THIS REDUCTION IN AVERAGE SPEED DECAYS (EXPONENTIALLY) WITH DISTANCE DOWNSTREAM. THERE IS A DISTINCT TIME HALO EFFECT. THAT IS, THE AVERAGE SPEED IS DEPRESSED FROM ITS PRE-ENFORCEMENT LEVEL AFTER ENFORCEMENT HAS BEEN REMOVED. FOR A SINGLE APPLICATION OF ENFORCEMENT, THE EFFECT SEEMS TO VANISH AFTER 3 DAYS. WHEN THE SPEED LIMIT AT A SITE IS ENFORCED FOR 5 CONSECUTIVE DAYS, THE AVERAGE SPEED REMAINS DEPRESSED AT LEAST FOR 6 DAYS AFTER THE LAST DAY OF ENFORCEMENT. IN TWO OF THE EXPERIMENTS, ENFORCEMENT WAS RELATED TO A REDUCTION IN THE "WIDTH" OF THE SPEED DISTRIBUTION. IN ONE EXPERIMENT NO SUCH REDUCTION OCCURRED. WHEN INDIVIDUAL VEHICLES ARE TRACED DAY AFTER DAY, IT APPEARS THAT REPEATED EXPOSURE TO ENFORCEMENT DOES NOT INDUCE LARGER REDUCTIONS IN THE SPEED OF TRAVEL. IT IS ALSO FOUND THAT HABITUALLY FAST AND HABITUALLY SLOW DRIVERS REDUCED THEIR SPEED SOMEWHAT MORE THAN THOSE DRIVING AT THE AVERAGE TRAFFIC STREAM SPEED. (Author/publisher).
Samenvatting