This category allows you to view and edit the sight line parameters.

Design Guidelines' Sight Lines Category 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.

Speed and Deceleration Section

Approach Speed

Specify the approach leg speed value of the design vehicle.

Perception-Brake Time

Specify the perception-brake reaction time value. This is the reaction time between the moment the driver sees an obstacle on the road and the moment they start applying the brakes. By default, the Perception-Brake Time value is set to 2.5 s.

Deceleration

Specify the deceleration rate value. This is the time rate of decrease in velocity with respect to the magnitude or direction. By default, the Deceleration value is set to 11.2 ft/s².

Sight Distance Type Section

Line of Sight / Stopping Sight Distance

Select how the minimum sight distance should be measured:

• Line of Sight is a straight line representing the sight of the driver. The line of sight is calculated based on a stopping sight distance equation and is measured from the driver's eye to the point of interest (e.g. to the crosswalk).
• Stopping Sight Distance is the distance required for a vehicle to completely stop from its initial speed, including the distance traveled within the perception-brake time.

Intersection Sight Distance Section

Distance to Yield Line

Specify the distance to yield line value. This is the setback distance from the conceptual yield line (inscribed circle) to the design vehicle when evaluating intersection sight distance.

Critical Gap

Specify the critical gap value. This is the duration of time needed for an entering vehicle to safely enter the roundabout without interfering with upstream or circulating stream vehicles. By default, the Critical Gap value is set to 6.5 s.

Driver's Eye Location Section

Distance from Front

Specify the distance from front value. This is the distance from the front of the design vehicle to the driver's eye

Distance from Driver's Side

Specify the distance from the driver's side value. This is the distance from the driver's side of the design vehicle to the driver's eye.