1. Unsignalized Intersections

2. 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

3. Rank of Movements 4 3 2 1

4. Gaps

5. 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?

6. Example (2/3) Gaps 4.5 6.2 8.3 10.4 Vehicles 1 2 3 Volume 20 3015
Left turns

7. Example (2/3) Gaps 4.5 6.2 8.3 10.4 Vehicles 1 2 Volume 30 151 2
Volume 30 15 10
Right turns

8. 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

9. 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

10. 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) = = 0.81