User Guide > Flow Modifiers

Flow Modifiers

The input turning flows can be modified by several flow modifiers.

Input flows are vehicles/hour capacity is derived in passenger car equivalents / hour (pcu/hr). Trucks are equivalent to 2 cars on roundabouts. The percentage trucks are therefore required in order to change veh/hr to pce/hr for capacity estimation.

A flow factor is provided to growth the arrivals turning flows up or down to different yearly flows or to alter them for any other particular reason. It is especially useful for varying flow levels in small steps to test the sensitivity of the results (queues and delays, etc.) to small changes in flows.

Both the Entry Start Queue and the Bypass Start Queue are the queues at the beginning of the peak hour. These are added to the hourly arrival flows to give the total demand flow on the entry during the peak hour.

When VC Ratios are low to medium, Start Queues are minimal and can be ignored. However at higher VC ratios the Start Queues can be significant and ignoring them can cause significant underestimates of delays, queues, etc.

Start Queues are known when modeling existing observed flows. However, when modeling future year flows they are not known and are usually ignored.

For future situations input zero rather than guessing the Start Queues and this will trigger a Start Queue estimation procedure that derives the Start Queues by modeling the pre-peak period.

The Start Queues are then added to the peak hour flows to give the total flow wanting to cross the yield line during the peak hour.

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