At-Grade Intersection Design

1. At-Grade Intersection Design CE 453 Lecture 32 For a nice overview of intersection design, see http://www.fhwa.dot.gov/environment/flex/ch08.htm

5. Design Vehicle Source: www\fhwa\Flexibility in Highway Design - Chapter 8 - FHWA.htm

6. Factors of influence: Design vehicle Intersection angle Approach width and parking Channelization Pedestrians Allowable speed reduction Curb Radius

7. Green Book Minimums assume: Turning Speed LTE 10 mph Vehicle properly positioned beginning and end Vehicle doesn’t cross inner edge of pavement Curb Radius

8. Radius Design • Simple curve • Low speed collector, local streets • Simple circle with taper • 3-centered compound curve

9. Curb Radius • General Guidance • 10 to 25 ft. local • 25 to 30 ft. collectors • 30 to 35 ft. unchannelized intersections with arterials Source: www\fhwa\Flexibility in Highway Design - Chapter 8 - FHWA.htm

10. Minimize lane encroachment R = 15 feet

11. Simple Curve (passenger car template)

12. Simple Curve with Taper (passenger car template)

13. Compound curves (passenger car template)

17. High Speed Turns

18. Intersection Design Considerations for Pedestrians

20. Provide median refuge to provide crossing in stages

21. Provision of crosswalks

22. Intersections on High Speed Rural Divided Expressways - Tom Welch, State Traffic Safety Engineer

27. High speed expressway intersections • We are building many rural expressways • At-grade intersections can be very dangerous • Don’t use traffic signals as a safety measure (in most cases) - leave as 2 way stop or replace with interchange • There are alternate measures …

28. Alternate measures • install double yellow center line and stop bars in the median • maintain and replace frequently • When resurfacing, look for opportunities to install more and longer left and right turn lanes • Paving shoulders near the intersections • Build "T“ intersections or even offset "T" intersections • much fewer conflict points • Use a restricted median • allow lefts in and rights in and out • prohibit lefts out and cross traffic

29. Alternate measures • ask local newspaper to publish an article which explains the proper way to cross and enter a divided roadway. • stop, look left only and enter into the median • stop in the median • look right and then cross or turn left onto the expressway • Wisconsin pioneered advisory speed limit 10mph below the posted speed • Signs need to be on both sides to work (where fast aggressive drivers operate) • In low volumes, traffic may not slow down • In peak hours (most Important because most turns), almost always one person in the platoon slows down - whole platoon slows

32. US 65 Bondurant Example • Speed study done before and after • Some problems with the speed plates • Intense enforcement during the before and after period • Data shows promise

34. Another idea … • median sign west on Boone on US-30 • Initially solar powered strobe light above the sign to draw attention • was distracting to drivers on US- 30

35. Change Blindness • a psychological phenomenon • In the past, motorists would pull up to a stop sign at a rural two lane highway and when they looked down the highway there would all kinds of buildings, fence posts tree etc along the narrow R/W and roadway • They could easily note the change in position of approaching vehicles and better judge the approaching vehicular closing speed. • Nowadays on our expressways, we cleanout the wide R/W, there are fewer buildings and trees along the R/W etc. • So, now there is nothing left to help the driver judge the change in position of the approaching vehicles on the expressway. • Imagine now a 75 year old driver with restricted eye sight, trying to judge the speed of a light colored vehicle up against the gray/ white horizon. • Tough to judge approaching speeds

36. Overhead intersection beacons? • Recent research in Minnesota • ineffective • confuse motorists (drivers may think the flashing red beacon means 4 way stop) • now removing over head flashing intersection beacons in favor of red beacon on the stop sign

37. Traffic circles • Rotaries: large diameter > 300 ft, allows speeds > 30 mph with minimum horizontal deflection of the path of through traffic • Neighborhood traffic circle: small diameter, for local streets, traffic calming • Roundabout • Yield control at each approach • Separation of conflicting movements • Speed < 30 mph (typically)