Signalized Intersection with Overlapped Left Turns
PLEASE - Read the Overlap Background page before continuing on. It will make things easier and less frustrating!
The object of this case is to find the optimal cycle length for the intersection of Case Study 3 using overlapped phases. How do the operations differ from Case Study 3?
Movement |
NBLT |
NBTH |
NBRT |
SBLT |
SBTH |
SBRT |
Volume |
360 |
868 |
66 |
231 |
748 |
50 |
Movement |
EBLT |
EBTH |
EBRT |
WBLT |
WBTH |
WBRT |
Volume |
96 |
146 |
80 |
105 |
133 |
98 |
| Desired v/c Ratio ((v/c)c) | 0.85 | Headway (h) | 2 sec |
| Peak Hour Factor | 0.90 | Lost Time per phase (tl) | 3 sec |
| Pedestrian Volumes | 200 per hour | Heavy Vehicles | 8% |
Worksheet:
For this case study, we have an Excel worksheet which you may use or print out. If you decide to use Excel, be sure to right-click the link and "save target as" or "save link as." If you are asked for a password, just press escape.
Step 1: Phasing Plan
Normally you would check to see if left turns are warranted, but that was already done in Case Study 3. Since we are using overlapped phases, our phase diagram will look like this:
Step 2: Volume Adjustment
Conversion of all volumes to tcus. The equivalent factors may be found here. Remember that ELT for protected left turns is 1.05.
Step 3: Critical Volume
Step 4: Cycle length
Step 5: Green Split
Check your green split values for Phase A1, Phase A1+2, Phase A2+3, Phase A3, Phase B.
Step 6: Signal Timing Plan
Signal Timing Plan includes the phasing plan, cycle length, and green splits.
Step 7: NEMA Phasing
In order to input the timing plan into the controller, you must assign NEMA phase numbers to the phases (convert Phase A to NEMA Phase #, and so on). The link above should help with this.
Step 8: Controller Settings
Follow all of the instructions given, especially the CLEAR and RESTART to ensure that your settings are properly implemented. Use the default all-red and yellow values (2 and 3 respectively).
When setting the lead/lag section in the controller, the phases in Ring 1 will need to be reversed. Also, it will be best to start both rings from Phase B from INIT (follow the controller settings link).
Step 9: CID Configuration
Download (if you haven't already) and open Case Study 4.trf
Step 10: Traffic Simulation
Use TSIS to open "NIATT Signal Timing Project - Case Study 4 - Simulation.trf"
Step 11: Execute Trafvu
This program allows you to view an animation of your signal. Do not start the animation until the simulation has reached equilibrium - usually about three minutes.
Questions
What do you notice about the simulation?
Does this seem to work better than the timing plan in Case Study 3?
Continue on to Case Study 5.