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QUEUING ANALYSIS
GENERAL
- Queue lengths can be determined by the following procedure:
1.25 x Volume = Q Length
- When large intersections are involved or it is known that a cycle length greater then 120 seconds is needed, the procedures described below will be used.
Procedures for determining queue lengths at signalized, and unsignalized intersections:
SIGNALIZED INTERSECTIONS
This procedure can be used at intersections with existing signals and intersections where it is felt a signal may be installed.
- Perform critical lane analysis
- Select Cycle length
- use existing timing if available
- if timing is not available, use the suggested cycle lengths
Recommended Maximum Cycle Lengths
LOS 2 Phase 3-5 Phase 6-8 Phase
A 90 100 120
B 90 100 120
C 100 120 135
D 120 135 150
E 135 150 165
F 150 165 180
NOTE: These cycle lengths are to be used as a guide, knowledge of the intersection may result in using a Higher or lower cycle.
- Use Poisson Distribution Chart/Formula to determine maximum number of vehicles per cycle of a specific movement.
FORMULA:
ritical Lane Volume (veh/hr) x Cycle Length(sec.)
Avg Veh/Cycle = ---------------------------------------
3600 (sec/hr)
- Assume a vehicle length of 25 ft.
- Once the average vehicles per cycle (specific movement) is determined, the chart can be used to find the maximum vehicles per cycle for that movement.
- The queue length will be the maximum vehicles per cycle times 25 ft. per vehicle.
- It is noted that the chart ends at an average of 20 vehicles per cycle. In cases where the average number of vehicles per cycle exceeds 20 the following formula can be used to determine the queue length. This formula can also be used in lieu of the chart.
Q. = Avg No. of Veh x 1.4 (Surge Factor) x 25 ft.
POISSON DISTRIBUTION
Average No. of Vehicle Maximum No. of Vehicle
Per Cycle Per Cycle
------------------ -------------------
0.1 - 0.3 1
0.4 - 0.8 2
0.9 - 1.3 3
1.4 - 1.9 4
2.0 - 2.6 5
2.7 - 3.2 6
3.3 - 3.9 7
4.0 - 4.7 8
4.8 - 5.4 9
5.5 - 6.1 10
6.2 - 6.9 11
7.0 - 7.7 12
7.8 - 8.4 13
8.5 - 9.2 14
9.3 - 10.0 15
10.1 - 10.8 16
10.9 - 11.6 17
11.7 - 12.4 18
12.5 - 13.2 19
13.3 - 14.0 20
14.1 - 14.9 21
15.0 - 15.7 22
15.8 - 16.5 23
16.6 - 17.3 24
17.4 - 18.2 25
18.3 - 19.0 26
19.1 - 19.8 27
19.9 - 20.0 28
UNSIGNALIZED INTERSECTIONS
This procedure can be used at isolated intersections where it is felt a signal will not be placed. If there is any chance that a signal may be placed at an intersection, the procedure for signalized intersections should be used.
- Determine the critical gap needed for the movement (from chart) this chart is also found in the 1985 HCM unsignalized intersections. BASIC CRITICAL GAP FOR PASSENGER CARS, SEC
Vehicle Maneuver Average Running Speed, Major Road
and 30 MPH 55 MPH
Type of Control Number of Lanes on Major Road
2 4 2 4
RT from Minor Road
STOP 5.5 5.5 6.5 6.5
YIELD 5.0 5.0 5.5 5.5
LT from Major Road 5.0 5.5 5.5 6.0
Cross Major Road
STOP 6.0 6.5 7.5 8.0
YIELD 5.5 6.0 6.5 7.0
LT from Minor Road
STOP 6.5 7.0 8.0 8.5
YIELD 6.0 6.5 7.0 7.5
Note: If restricted sight distance exists add one second to the gap needed. Where average running speeds are between 30 mph and 55 mph, interpolate.
- Determine average gap between opposing vehicles
Average Gap Opposing Vehicle 3600 sec / (Volume Per/Hour)
- If the average gap is greater than the gap needed for the maneuver the same procedure as signalized intersections can be used with the cycle length equal to the critical gap required (from chart) plus 4 seconds (start up time).
- If the average gap is less than or equal to the gap needed, this maneuver should be analyzed as if a signal were in place.
No Lanes Factor
1 1.00
2 .55
3 .40
4 .30
DBL L.T .60
NOTE: When traffic impact studies are being prepared in jurisdictions where lane use factors are part of Adequate Public Facilities Ordinance (A.P.F.O.) or established guidelines those factors may be used to analyze SHA intersections.
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