User Guide > Traffic Flows

Traffic Flows

Rodel stores and uses Peak hour turning flows (veh/hr) for the AM peak, Off Peak and PM Peak. The turning flows include Bypass flows. A peak is selected using the dropdown menu.

For driving on the right (RHD), the legs of the roundabout are numbered counter-clockwise. The first leg can be chosen arbitrarily, but is helps to reduce mistakes if a convention is adopted such as leg 1 = the north leg (southbound entry).

The leg number gives each entry and each exit an absolute number. A turning movement is the flows from an entry to an exit. The entry of a turn is specified by the leg number.

However, the exit of a turn is not specified by leg number of the exit, as using the exit number is confusing and leads to input error and errors reading the turns.

Instead the exit is defined relative to the entry. This is the experience of a driver using the roundabout. The driver approaches and makes a right, through or left turn relative to the entry used by the driver. This is easy to visualize and it stores each turn type in a column.

In the above Arrival Turning Flows table (RHD), row 1 is entry leg 1 and row 4 is entry leg 4. The right turns are in the Exit-1 column. Exit-2 is through traffic. Exit-3 is left turns and exit 4 is U turn traffic. The bypass traffic is in a separate column.

Leg 2 through traffic = 60 v/hr Leg 4 left = 150 v/hr

The Traffic Flow Profile

The turning flows summate to the total hourly arrival flows on each leg. For example the turns on leg 4 above 160, 150, 140, 130 (0) sum to an arrival flow of 580 v/hr.

Besides being the total hourly flow, it is the average flow rate expressed as veh/hr. It could equally be expressed as 9.67 veh/min or 0.161 veh/sec.

However, traffic rises and falls during the peak hour so the flow rate varies and using the average value is too coarse for assessing queues and delays. Consequently it is necessary to convert the average hourly flows into a traffic profile that represents the variation in the flow rate during the peak hour under consideration.

There are three alternative methods, selected by the Flow Profile drop down.

1. Direct Flow Profile

The Direct Flow option allows observed flows to be entered for each time interval. The observed traffic profile below shows traffic counted in 5-minute Time Slices.

Time Slices (minutes) are selected using the drop down.

This can only be done when modeling an existing situation. As the actual rise and fall can be observed and measured there is no need to estimate the shape of the profile from average hourly flow rates.

For future situations the average peak hour arrivals are estimated and then have to be reshaped into an estimate of the peak hour flow profile. Two methods are provided. The first is using a Peak Hour Factor to derive a course peak hour profile or the second method can be used to synthesize a more detailed profile.

2. Peak Hour Factor Flow Profile

The Peak Hour Factor (PHF) uses a single flow ratio and single implied flow time.

A PHF of 0.9 is shown below for each leg.

This creates a central 15-minute peak flow rate that is the average rate divided by 0.9.

The "shoulders" either side of the 15-minute peak are reduced so that the total hourly flow is unchanged.

This profile is divided into 8 Time Slices of 7.5 minutes.

3. Synthetic Profile

The Synthetic method uses three Flow Ratios and three Flow Times to create a more detailed and flexible profiles that give a more realistic estimate of the traffic arrival rate.

The typical profile shown above extends over the whole hour. However, the Flow Times can be set other than 0, 30 and 60 to create any shape. For example the profile for the short intense discharge from a school or factory can easily be created. The profile can also be skewed by moving the Time 2 off center.

The summary below compares an observed profile with the PHF and Synthesized profiles.

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