Turbo Roundabout Envelopes Design Method

Add approach legs to the roundabout, and edit approach legs' parameters.

Note: This tool requires existing CAD geometry in the drawing to act as the leg reference geometry. The geometry may consist of lines, arcs, or polylines/complex chains. Lines and arcs can be combined within the polylines/complex chains.

Add Leg / Edit Leg Dialog Box

In the dialog box, move the mouse pointer over the controls to view the associated tooltip and then reference the controls described below.

Note: For the purposes of describing the software, it is assumed that Right-Hand Traffic is selected under Traffic Direction in Design Guidelines' General Category. If Left-Hand Traffic is selected, the descriptions with references to a right/left turn path should be interpreted in the opposite direction.

Note:

• Legs won't draw until the specified number of spiral starts have been added to the turbo roundabout which means that if the Number of Spiral Starts is 3, three reference elements need to be picked in the drawing before the legs are drawn.
• All options may not be available until legs on either side of the selected leg have been added.

Name

Specify a name for the approach leg.

Spiral Properties Section

Spiral Start

Select this check box to move the existing spiral start in the sector to reference this leg instead. A spiral start is where a circulating lane starts.

Inner Lane Start

Select the number of the circulating lane that the leg's inside entry lane begins circulating the turbo roundabout. When the leg does not have a spiral start, select the desired circulating lane in the roundabout that the inside entry lane goes to. When the leg has a spiral start, the inside entry lane goes to the inner circulating lane.

Spiral Exit Leg

Select the approach leg name of the leg where the principle spiral movement exits the turbo roundabout. This control is only available when the leg has a spiral start.

Revolutions

Select the number of revolutions within which the spiral movement exits the roundabout. For example, when Revolutions is set to a value of 1, the spiral movement exits the roundabout within the first revolution. This control is usually used in conjunction with Spiral Exit Leg option to control the desired maneuver of the spiral movement.

 

Lane Widths Section

Select how the lane widths are drawn. Different controls are available in the Edit Leg dialog box depending on which mode is selected:

• When Effective Lane Widths is selected, the Splitters & Crosswalks section displays controls to trim the splitter island envelope by minimum width or length. The Lane Taper Ratios section is available to adjust the taper ratios of the entry and exit lanes.
• When Fixed Lane Widths is selected, the Splitters & Crosswalks section displays controls to adjust the median. The Road Widening section is available to adjust the approach and departure tapers.

Effective Lane Widths

Select this option to have lane widths determined by adding the entry and exit lane offsets specified in the current Design Guidelines' Leg Offsets Category to the width of the vehicle’s off-tracking as it enters and exits the roundabout.

Fixed Lane Widths

Select this option to apply the fixed lane widths specified in the current Design Guidelines' Fixed Lane Widths Category.

Properties Section

Button	Option	Click to...
	Draw Entry Lanes	Turn the approach leg's entry lane on/off. By default, the approach leg's entry lane is turned on. The entry path radius value is displayed in the box on the right. Specify the number of lanes for the approach leg entry.
	Configure Entry Path Radius and Offsets	When Effective Lane Widths is selected under Lane Widths: Override the entry path radius and offsets specified in the current Design Guidelines' Leg Arc Radii Category and Design Guidelines' Leg Offsets Category. When Fixed Lane Widths is selected under Lane Widths: Override the radius and fixed lane width settings. Note: This button is only available if an entry lane is turned on.
	Draw Exit Lanes	Turn the approach leg's exit lane on/off. By default, the approach leg's exit lane is turned on. The exit path radius value is displayed in the box on the right.
	Allow Inner Lane to Exit	To allow the inner lane closest to the outer lane to exit the roundabout at the selected Spiral Exit Leg. By default, this feature is turned on. To disallow exiting of the inner lane at this approach leg, click this button. The exit path radius is displayed in the box on the right.
	Configure Exit Path Radius and Offsets	When Effective Lane Widths is selected under Lane Widths: Override the exit path radius and offsets specified in the current Design Guidelines' Leg Arc Radii Category and Design Guidelines' Leg Offsets Category. When Fixed Lane Widths is selected under Lane Widths: Override the radius and fixed lane width settings specified in the current Design Guidelines' Fixed Lane Widths Category. Note: This button is only available if an exit lane is turned on.
	Preview Entry/Exit Path Radii	Turn the approach leg's entry/exit path radii preview on/off. By default, the approach leg's entry/exit path radii preview is on.
	Draw Mountable Curb	Turn the mountable curb on/off. The mountable curb is drawn initially based on the setting in the current Design Guidelines' Mountable Curb Category.
	Configure Mountable Curb	Override the mountable curb settings specified in the current Design Guidelines' Mountable Curb Category. Note: This button is only available when the mountable curb is turned on.
	Draw Bypass Lane	Turn the bypass lane on/off. The bypass lane connects the outside entry lane of the currently selected approach leg to the outside exit lane of the next downstream approach leg. By default, the bypass lane is turned off. The inscribed circle to bypass lane offset is displayed in the box on the right.
	Configure Bypass Lane	Override the bypass lane settings specified in the current Design Guidelines' Bypass Lane Category. Note: This button is only available if the bypass lane is turned on.

Leg Length

Specify the length of the selected approach leg.

Circulating lane Opening Widths Section

Override the settings specified in the current Design Guidelines' Circulating Lane Opening Widths Category for how the circulating lane opening widths are drawn at this leg of the turbo roundabout.

Opening Width Vehicle

Select this option to have the opening width determined by the tire track offset of the Opening Width design vehicle path. The Opening Width design vehicle follows a tighter turning maneuver as it enters the circulating lanes than normal design movements.

Button	Option	Click to...
	Configure Opening Width Vehicle Path	Override the Opening Width design vehicle's entry path radius and the tire track offset.

Opening Width

Select this option to have the opening width based on a specified width.

Splitters & Crosswalks Section

Button	Option	Click to...
	Draw Splitter Island Envelope	Turn the splitter island envelope on/off. By default, the splitter island envelope is on. Note: If you turn the splitter island envelope off, the Draw Pedestrian Refuge and Configure Splitter Island Envelope buttons will be unavailable.
	Draw Pedestrian Refuge	Turn the pedestrian refuge on/off. By default, the pedestrian refuge is on. Note: This button is only available if Draw Splitter Island Envelope is on.
	Draw Crosswalks	Turn the crosswalks on/off. By default, the crosswalks are on. Note: If the entry or exit crosswalk offset cannot be calculated, the crosswalk will be drawn across the leg using the other calculated offset.
	Configure Splitter Island Envelope	When Effective Lane Widths is selected under Lane Widths: Override the splitter island envelope offsets specified in the current Design Guidelines' Splitter Island Envelope Category and specify a Median Width or Minimum Refuge Width. When Fixed Lane Widths is selected under Lane Widths: Override the Inscribed Circle to Splitter Island Envelope Offset only. Note: This button is only available if the splitter island envelope is turned on.
	Configure Crosswalk Offsets	Override the crosswalk offsets specified in the Design Guidelines' Refuge/Crosswalk Category. Note: This button is only available if the crosswalks are turned on.

Trim Splitter Island Envelope

Note: The Trim Splitter Island options are only available for Effective Lane Widths approach legs.

Select this check box to trim the splitter island envelope by:

• Minimum Splitter Width: Trim when narrower than this value.
• Splitter Island Length: Trim the envelope to this length value.

Median

Note: The Median options are only available for Fixed Lane Widths approach legs.

Select this check box to extend the splitter island envelope to include a median island envelope.

Width

Specify the width of the median island envelope.

Continuous Median

Select this option to specify a continuous median island envelope for the full length of the leg.

Introduced Island

Select this option to specify an introduced island envelope. The Approach Taper and Departure Taper settings determine how the end of the introduced island envelope is drawn (see Road Widening Section below).

Width at Tie-in

Select this option to specify a continuous median island envelope whose width at the splitter and tie-in point can be specified.

Lane Taper Ratios Section

Note: The Lane Taper Ratios Section is only available for Effective Lane Widths approach legs.

Specify the position of the approach leg's entry/exit lane side relative to the reference element clockwise or counterclockwise. This option is useful if you need to match the roundabout design with the last curve of an S-shaped leg entry/exit. For example, if the counterclockwise 1:8 entry lane taper ratio is applied, the approach leg's entry lane side will taper at 1 unit to the right for every 8 units towards the reference element. If the clockwise 1:8 entry lane taper ratio is applied, the approach leg's entry lane side will taper at 1 unit to the left for every 8 units towards the reference element.

CW

Clockwise from the reference element.

CCW

Counterclockwise from the reference element.

Road Widening Section

Note: The Road Widening Section is only available:

1. for Fixed Lane Widths approach legs and
2. when, under Median, the Introduced Island option is selected, or the Width at Tie-in option is selected and the Width at Tie-in value is less than the Width value.

Approach

Specify the approach taper ratio value and the approach taper starting distance value. The approach taper moves traffic laterally to the right (as traffic approaches the roundabout) based on the approach taper ratio value. The purpose is to provide space for a left-turn lane.

Departure

Specify the departure taper ratio value and the departure taper starting distance value. The departure taper directs traffic to the left (as traffic departs the roundabout) based on the departure taper ratio value. Normally, the departure taper begins opposite the beginning of the full width turn lane and continues to a point opposite the beginning of the approach taper.

Calculated Output Section

Expand this section to view the widths of the entry, entry and exit lanes, the median, and the refuge as well as the entry angle measured using Method 1 and Method 2 (see Appendix -> Entry Angle Measurement).