1. At-Grade Intersection Design
CE 453 Lecture 32
For a nice overview of intersection design, see

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

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

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

8. Radius Design Simple curve Simple circle with taper
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)