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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.

# Left TurnService Required?

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.

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 6511026265 6511291 SB SBLT SBTH SBRT 31 374 50 14.36 1.00 1.32 1.08 1.08 1.08 481 404 71 481475 EB EBLT EBTH EBRT 116 466 125 2.86 1.00 1.32 1.08 1.08 1.08 358 503 178 358681 WB WBLT WBTH WBRT 52 292 80 4.91 1.00 1.32 1.08 1.08 1.08 276 315 114 276429

# 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 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).