Commentary 8 SCATS Degree of Saturation
DS in the SCATS traffic signal control system is an abbreviation for degree of saturation. It is formulated as shown in Equation C1 (Austroads 2007d).
|DS = (GT – Sact + n SMF ) x 100%/GT||C1|
|GT||=||is the green time (s)|
|Sact||=||is the total space time in that green period (s)|
|SMF||=||is the space at saturation or maximum flow (s/veh)|
|n||=||is the number of spaces counted (veh)|
Equation C1 gives the ratio of green time used by vehicles to the available green time, a measure of how well a green period is being used. It is not precisely the same as the usual definition of degree of saturation in traffic flow theory, which is the arrival flow or demand divided by the capacity. It should be noted that true demand is difficult to measure with a point sensor especially if the sensor is at the stop line. SCATS is able to estimate oversaturation by choosing an appropriate length for the inductive loop sensor.
The DS equation has been effective as a feedback control parameter in optimising signal phases and cycle times. DS is also a reasonable indicator of congestion including the situations when DS is greater than 1.
To identify over-saturation due to recurrent congestion or incidents, SCATS uses DS and two traffic volumes: Vo the number of vehicles recorded in a green period, and Vk the reconstituted volume in that green period, given by Equation C2:
|Vk = DS x GT x MF||C2|
|MF||=||the maximum flow (veh/s)|
The identification of congestion is specified in Figure C8 1. The ratio Vk/Vo must exceed 2.4 and the DS must exceed 0.95. This top right-hand quadrant represents situations when the system reliably indicates congestion.
Source: Austroads (2007c).