CE 578 Highway Traffic Operations Lecture 2 and 3: Vertical Alignment and Sight Distance

Objectives Vertical curve design parameters Sight distance calculation Sight distance measurement Sight distance based design speed calculation

Vertical Curve Design Parameters G 1 and G 2 grade at tangents 1 and 2 (%) Achange in grade (%) Eexternal distance (ft) yvertical offset from tangent 1 to point P (ft) Lvertical curve length (ft) xhorizontal distance from VPC to point P (ft) Parbitrary pointy on curve

Vertical Curve Design Parameters (cont.)

Vertical Curve Equations

Sight Distance Calculation Crest vertical curve –S <= L –S >= L Solve for S For passing conditions, usually sag curves not an issue –Day-time conditions provide full sight of curve –Night time conditions can see head lights of on-coming vehicles

Sight Distance Calculation (cont.) Horizontal curve (solve for S, use degrees) –Rradius from center of curvature to the middle of inside lane –Mmiddle ordinate or distance from sight obstruction to middle of inside lane

Horizontal Curve Sight Distance Measurement Straight edge Location of subject station Centerline intersect point Sight obstruction (3.5 ft for passing) (2.75 ft for stopping) S

Vertical Curve Sight Distance Measurement Limit of passing sight distance On-coming vehicle driver eye height (h 2 ) Driver eye height (h 1 ) Tangent 1 Tangent 2 S

Sight Distance Operational Effects Passing limitations –Driver ability to pass –Passing zone design Passing zone length Frequency –Passing lane design Driver anticipation of beginning Speed limitations

Assignment Problem 1 USE PAGE 2 OF ASSIGNMENT 1 Calculate vertical curve design parameters –Given L elevation of VPC station of VPC G 1 G 2 –Find elevation and station of VPT and VPI At VPC curve At VPC curve

Assignment Problem 2 USE PAGE 2 OF ASSIGNMENT 1 Calculate and measure the sight distance assuming middle ordinate of 25 ft for: –PC curve –PC curve Calculate and measure the sight distance for: –VPC curve