Unsignalized Intersections

Basic Terminology Two-Way Stop All-Way Stop Rank Movement Critical gap Stop on minor approaches All-Way Stop Stop on all approaches (4-WAY) Rank Movement Priority along allowable movements Critical gap Minimum acceptable time to complete a movement Follow up gap Additional time for next vehicle to complete same movement

Rank of Movements 4 3 2 1

Gaps

Example (1/3) EB: 20 gaps of 4.5 sec, 30 gaps of 6.2 sec, 15 gaps of 8.3 sec, and 5 gaps of 10.4 sec How many WB left turns? NB right turns?

Example (2/3) Gaps 4.5 6.2 8.3 10.4 Vehicles 1 2 3 Volume 20 30 15 Left turns

Example (2/3) Gaps 4.5 6.2 8.3 10.4 Vehicles 1 2 Volume 30 15 1 2 Volume 30 15 10 Right turns

Gap Distribution Arrivals Time headway Poisson P(x) = mx e-m/x! where m arrivals in time period Time headway P(h>t)= e-t/T where T average headway; t given headway

Example 1 Left turn average 3 veh/min; failure rate 20% Length of left turn bay? X 1 2 3 4 5 P(X) 0.05 0.15 0.22 0.17 0.10 ΣP(X) 0.05 0.20 0.42 0.64 0.81 0.91 For 4 vehicles

Example 2 Approach volume 2000 vph Gaps > 3 sec? Gaps < 3 sec? 1999 T = 3600/1999 = 1.80 sec/veh P(h>3)= e -3/1.80 = 0.19 P(h<3)= 1- P(h>3) = 1- 0.19 = 0.81