CAPACITY STUDY OF SIGNALIZED DIAMOND INTERCHANGES

THE RESULTS OF A RESEARCH PROJECT ON FREEWAY RAMPS AND INTERCHANGES WHICH IS BEING CONDUCTED BY THE TEXAS TRANSPORTATION INSTITUTE IN COOPERATION WITH THE TEXAS HIGHWAY DEPARTMENT ARE PRESENTED. THE STUDY WAS DESIGNED TO OBTAIN TRAFFIC PERFORMANCE DATA WHICH WOULD HAVE USEFUL APPLICATION IN EVALUATING THE CAPACITY OF SIGNALIZED DIAMOND INTERCHANGES. LIBRARY RESEARCH AND EVALUATION OF EXISTING DESIGN REQUIREMENTS INDICATED A GREAT DEMAND AND NEED FOR THIS TYPE OF DATA. THE FIELD DATA WERE GATHERED THROUGH THE USE OF TIME-MOTION PICTURES WHICH FURNISHED A COMPLETE AND SIMULTANEOUS RECORD OF THE TRAFFIC OPERATIONS OCCURRING IN THE INTERSECTIONS THAT WERE STUDIED. THE OPERATIONAL CHARACTERISTICS OF OVER 4,000 VEHICLES WERE RECORDED ON 16-MM FILM AT TWO CONVENTIONAL TYPE DIAMOND INTERCHANGES ON THE GULF FREEWAY IN HOUSTON, TEXAS. THESE DATA WERE COLLECTED DURING THE PEAK PERIODS ON APPROACHES THAT WERE FULLY LOADED. THE RESEARCH WAS CONDUCTED TO INVESTIGATE THE FACTORS WHICH COULD BE USED IN DEVELOPING CRITERIA FOR EVALUATING THE CAPACITY OF THE VARIOUS MOVEMENTS ENCOUNTERED IN DIAMOND INTERCHANGE OPERATION. DATA ON VEHICLE STARTING DELAYS AND TIME-HEADWAYS AT BOTH INTERCHANGES WERE ACCURATELY MEASURED TO DEVELOP A BASIC APPROACH TO THE DETERMINATION OF LANE CAPACITY. SPECIAL EMPHASIS WAS PLACED ON DETERMINING THE CAPACITY OF A SINGLE-LANE TURNING MOVEMENT AND A TWO-LANE, OR TWO-ABREAST-TYPE TURNING MOVEMENT. THE ANALYSIS OF THE STUDY PRODUCED SOME SIGNIFICANT RESULTS AND PROVIDES THE DESIGNER WITH CURRENT OPERATIONAL CHARACTERISTICS OF VEHICLES AT SIGNALIZED DIAMOND INTERCHANGES. DESIGN PROCEDURES FOR DIAMOND INTERCHANGES, BASED ON LANE CAPACITY AND SIGNALIZATION REQUIREMENTS, WERE DEVELOPED AND ARE PRESENTED IN THIS REPORT. /AUTHOR/