Geometry (CL Inside Lane)


Specific Station Calculator

Horizontal Sight Distance

This program determines if an object (barrier wall, guardrail, cut-slope, retaining wall, bridge pier, plant, building, etc.) blocks the line of sight distance required in a simple curve. This is covered by the Green Book (AASHTO 2018) Equation 3-37, and is exact when the object and vehicle are both inside the curve. But isn't correct when the horizontal sightline offset is near the ends of the curve. This program includes an iterative module that solves for the required HSO throughout the entire curve by checking intervals of .00001 feet. Then the program compares the HSO avaliable to the required HSO to determine if there is sufficient sight distance clearance.

The stationing at the centerline of the inside lane should be customized for the conditions and is not the roadway baseline geometry in many cases. The program assumes that the stationing is in the direction of travel, but that isn't always the case. For example, the roadway may have two way traffic with one way in the direction of stationing and the other way opposite of the direction of stations. Even in one-way cases the direction of travel is often opposite the direction of stationing. Where this occurs, the solution at this time is to develop a custom alignment with stations in the direction of travel.

Input variables:


Criteria Basis:

FDOT Interstate - Stopping Sight Distance

FDOT Other - Stopping Sight Distance

AASHTO SSD - Stopping Sight Distance

Grade in % (SSD Only) - Use steepest downgrade for greatest SSD

Select Tables or Equations radio button

AASHTO DSD - Decision Sight Distance

Select Maneuver Type:

A - Stop, Rural Area

B - Stop Urban Area

C - Maneuver, Rural Rd

D - Maneuver, Suburban Rd

E - Maneuver, Urban Rd

Design Speed:

Select the Design Speed from the dropdown options after selection of the Criteria Basis

Geometry (CL Inside Lane)

Radius R - Radius at the centerline of the inside lane in feet

Delta DMS - Delta angle of the curve in Degrees Minutes Seconds

PC Station - The PC Station along the centerline of the inside lane

Object Station - This is the Station along the centerline of the inside lane where the sight obstruction/object is offset>

HSO Avaliable - This is the horizontal sightline offset (feet) between the centerline of the inside lane and the sight obstruction>