In oversaturated cases, demand exceeds capacity, i.e. the degree of saturation is expected to be larger than 1.0. However, when only stop or give-way / yield line volume counts have been used as input to the model since the demand volumes as measured at the back of queue are not available, the estimation of performance statistics (delay, queue length, etc) is problematic. The following method involving estimation of demand flows is recommended in this case.
- Firstly adjust the key calibration parameters summarised in User Guide, Section 2.6 in order to obtain lane degrees of saturation that equal 1.0 approximately, i.e. lane capacities equal observed lane flows based on stop or give-way / yield line counts specified as input.
- With this calibration, the queue lengths, therefore probabilities of blockage, and delays will be underestimated since the stop or give-way / yield line volume counts are lower than the actual demand volumes. Use the Flow Scale parameter in the Volumes dialog, Volume Factors tab (User Guide Section 5.8.2) to estimate the demand flow rates. Select Flow Scale values that reflect the observed queue and delay conditions. As a result, the lane flow rates will be higher than those based on input volumes.