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Improved Signal Timing - Case Study 1
In this portion of Case Study 1, we will be calculating the optimal cycle length for this intersection. Once the analysis software is running, we will be able to qualitatively compare the signal timing plan with the original one.
Movement |
NBLT |
NBTH |
NBRT |
SBLT |
SBTH |
SBRT |
Volume |
186 |
950 |
186 |
31 |
374 |
50 |
Movement |
EBLT |
EBTH |
EBRT |
WBLT |
WBTH |
WBRT |
Volume |
116 |
466 |
125 |
52 |
292 |
80 |
| 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
A major factor in determining a phasing plan is the service provided to left turn movements (i.s. is a protected left turn phase needed?). There are two basic criteria for this:
Is the volume of left turners more than 200 vph? If so, a protected left can be justified.
Is the product of the through opposing volume per lane and the left turn volume greater than 50,000. If this product is larger than 50,000 then a protected left turn phase may be needed.
Movement |
Turn Volume < 200? |
Product < 50,000? |
Left Turn |
| NB | Vlt = 186 < 200 vph | 186 · (374/2) = 34782 < 50,000 | No |
| SB | Vlt = 31 < 200 vph | 31 · (950/2) = 14725 < 50,000 | No |
| EB | Vlt = 116 < 200 vph | 116 · (292/2) = 22192 < 50,000 | No |
| WB | Vlt = 52 < 200 vph | 52 · (466/2) = 12116 < 50,000 | No |
No protected left turns are warranted so we will consider this intersection as a two phase signalized intersection.
Step 2: Volume Adjustment
Vehicles that are turning will cause more congestion than cars that are going straight. This is generally because they must slow down to go around the corners, and in the case of left turners, there are situations in which they block through lanes waiting to turn. Therefore we use factors to convert the turners into through car equivalents (tcus). Those factors can be found here. We must also adjust the volumes for the heavy vehicle volumes, since large trucks affect traffic operations more than passenger cars.
Our heavy vehicle factor will be calculated as follows:
Now you calculate equivalent factors and the volume in tcus for this problem.
|
Approach |
Movement |
Volume (vph) |
Equivalent Factor |
Heavy Vehicle Factor (1/fHV) | Volume (tcus) |
Shared Lane Group (tcus)** |
| NB |
NBLT NBTH NBRT |
186 950 186 |
3.24 1.00 1.32 |
1.08 1.08 1.08 |
651 1026 265 |
651 1291 |
| SB |
SBLT
SBTH SBRT |
31
374 50 |
14.36
1.00 1.32 |
1.08 1.08 1.08 |
481
404 71 |
481 475 |
| EB |
EBLT
EBTH EBRT |
116
466 125 |
2.86
1.00 1.32 |
1.08
1.08 1.08 |
358
503 178 |
358 681 |
| WB |
WBLT
WBTH WBRT |
52
292 80 |
4.91
1.00 1.32 |
1.08
1.08 1.08 |
276
315 114 |
276 429 |
Step 3: Critical Volume
The figure shows the per-movement tcus values, with left turn values in blue. Using the figure and the information in the Critical Volume link, determine the critical volume for this intersection. Remember the through movements each have two lanes!
Step 4: Cycle length
Step 5: Green Split
Check your green split values for Phase A and 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 pull down menus on the table above show which NEMA numbers could apply to the phase.
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).
Step 9: CID Configuration
Download (if you haven't already) and open Case Study 1.trf
Step 10: Traffic Simulation
Use TSIS to open "NIATT Signal Timing Project - Case Study 1 - 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.
Continue on to Case Study 2.