Traffic Progression Assessment in Traffic Impact Studies

T impact studies have been widely used to identify and manage urban traffic growth. Many state and lcJcal government agencies rely on the information contained in traffic impact studies when considering a proposed development and when determining the extent of roadway improvements necessary to mitigate any negative traffic impact generated by the development. Most traffic impact studies use the concept and methodology presented in the 1985 Highwuy Cupacity Munual (HCM)’ to determine the degree of deterioration on the surrounding roadway system, in terms of level of service (LOS). For signalized intersections, the HCM methodology determines LOS based on average vehicular delay. For urban and suburban signalized arterials, the HCM methodology uses the average travel speed to determine the LOS for the arterial through traffic. The average travel speed is computed from the running time between two signals, plus the approach delay at the downstream intersection. The approach delay is computed from the average vehicular delay using the methodology for signalized intersections. The average vehicular delay at signalized intersections is therefore a key element in determining the roadway LOS. When several traffic signals in close proximity are coordinated, vehicular delay and LOS will be significantly affected by the quality of traffic progression. When traffic progression is favorable to the subject traffic flow (i. e., most vehi cles arrive in the green time), delay will bc considerably less than that for random arrivals (i.e., vehicles arrive equally in both the red and green times). Conversely, when traffic progression is unfavorable, (i. e., most vehicles arrive in the red time), delay can be considerably higher than that for random arrivals. The influence of traffic progression on delay can be identified by simultaneously considering the signal control at the two adjacent intersections and the traffic flows from the upstream intersection to the downstream one. The HCM, however, treats traffic progression in a different manner because the HCM methodology is unable to simultaneously analyze two intersections. The HCM uses a progression adjustment factor (PF) to account for the influence of traffic progression. Delay for each movement is originally computed without considering progression. A final progression adjustment is then performed by multiplying the delay value and the PF value. The progression adjustment can increase the delay by up to 85 percent for the worst progression and decrease the delay by up to 60 percent for the most fworable progression. The values of PF provided in the HCM are primarily based on arrival types. Vehicle arrivals at a signalized intersection are classified into five types. Types 1 and 2 represent the worst conditions, when most vehicles arrive during the red interd. Conversely, types 4 and 5 represent the most favorable conditions, when most vehicles arrive during the green interval. Type 3 represents the random arrival condition, when vehicles arrive uniformly during both the red and green intervals. Based on the definition of arrival type in the HCM, arrival type is difficult to assess subjectively. To cope with this difficulty, the HCM suggests the use of platoon ratio to quantify the arrival type. A relationship between arrival type and platoon ratio is provided in ~dble 9-2 of the HCM. The platoon ratio is defined in the HCM as