Roundabouts Properties: Fastest Path Analysis: Fastest Path Calculation

One of the factors used to assess the safety of a roundabout is the maximum realistic speed of a notional vehicle on the most direct line (the fastest line) through the roundabout. This is known as the fastest line speed. To calculate the fastest line speed, Roundabouts first calculates the required alignment and then steps along it testing for the smallest radius. The smaller the radius, the slower the speed. The speed is calculated using the following equation which is taken from the AASHTO document ‘A Policy on Geometric Design of Highways and Streets’:-

V² = 127R(e+f)

Where:

• R is Turn Radius (m)
• V is Speed (km/h)
• e is Super-elevation (assumed to be 0.0)
• f is Side Friction Factor (adjusted for speed)

Expand Fastest Path Analysis and click Fastest Path Calculation in the left hand pane. The Fastest Path Calculation page is displayed.

The method by which the fastest line is calculated varies and differing methods often give different answers. The settings on this page let you adjust how the calculation is done.

The following offsets may be measured from definition lines or curb lines and from island or apron. Use the Offset Path From drop down list to select the required method.

Note: In some parts of the world (notably the USA) the apron is considered to be a part of the center island. In these cases the fastest path would normally be offset from the center island and not the apron. In other areas where the apron is considered to be outside the island, the fastest path would normally be offset from the apron and not the center island.

• The Start Distance from Yield Line is the straight line distance from the inscribed circle to the front axle of the test vehicle.
• The Start Centerline Offset is the minimum offset from the nearside or offside curbs or definition lines.
• The Entry Offset is the minimum offset of the centerline of the vehicle from the nearside entry curb or definition line.
• The Island Offset is the minimum offset of the centerline of the vehicle from the island or apron.
• The Exit Offset is the minimum offset of the centerline of the vehicle from the nearside exit curb or definition line.
• The End Centerline Offset is the minimum offset from the nearside or offside curbs or definition lines.

The End Distance from Exit Line is the straight line distance from the inscribed circle to the front axle of the test vehicle.

The Radius Calculation Length is the length over which the radius should be tested. Roundabouts calculates the radius of a 3-point arc of this length along the fastest line using the start, middle and end points.

The fastest line and speed are calculated continuously and optionally displayed in the HUD or Roundabout Report.

In all cases the Max and Min values represent the limiting values from the adopted design standard.