Critical Clearance Geometry

Lower Roadway Geometry

SSD Calculations

Sight Distance at Undercrossings

This program determines if a bridge (or obstruction) over a roadway cuts the line of sight distance required. The Green Book (AASHTO 2018) Figure 3-38 provides an approximation (and sometimes not approximate) solution to this problem. The Green Book method is calculated but the program also includes an iterative method to better solve for the clearance between the sightline and the roadway profile at the critical station. A check is made at intervals of 0.0001 feet between (Critical Station - Sight Distance) and the Critical Station to find the greatest sightline elevation at the Critical Station with greater accuracy than the AASHTO method. Then the program compares the minimum clearance (between the overhead obstruction and the profile) to the sightline clearance(between the sightline and the roadway profile at the critical station) to determine if the sufficient clearance.

The vertical geometry of the lower roadway 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 profile with stations in the direction of travel.

There are other reasons a custom profile may be needed. The profile should pass through the point where Minimum Clearance is calculated or an adjustment to the Minimum Clearance may be needed. Also the the Eye Height and the Object Height should be from the center of the lane and these may also need to be adjusted. And the horizontal geometry of the lower roadway may be in curve where the sightline is less than the sight distance to maneuver or stop.

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

AASHTO PSD - Passing Sight Distance

Design Speed (mph):

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

Critical Clearance Geometry

Eye Height - Height of a vehicle (truck) in feet and defaults to 8 feet

Object Height - Height of the object in feet and defaults based on the Criteria Basis

Critical Station - Station where the obstruction is closest to the profile. The Station where the Minimum Clearance occurs

Min. Clearance - This is the distance between the obstruction and the profile in feet at the Critical Station

Low Beam Elevation - The elevation of the low beam is optional if the Min. Clearance isn't known

Note: The Min. Clearance is calculated as the distance between the low beam (or obstruction) at the Critical Station

Lower Roadway Geometry

Note: The lower roadway geometry must define a sag vertical curve where G2 - G1 is positive

VPI Station - Station of the lower roadway Vertical PI

VPI Elevation - Elevation of the lower roadway Vertical PI

VC Length - Length of the Vertical Curve in feet

G1 % - the beginning grade in %

G2 % - the ending grade in %