CAPACITY PROCEDURES INTERSECTIONS
CRITICAL LANE ANALYSIS
This section describes the procedures for the capacity analysis of intersections. They include some revision but rely heavily on early work in this area by McInerney and Petersen, and later efforts by JHK and Associates and material contained in the 1985 HCM. The methodology described will fit most intersection configurations and can be varied easily for special situations and unusual conditions. This method applies at isolated intersections or any other location where the operation is not radically affected by adjacent traffic signals. Modifications to this procedure or use of methods suggested for arterials are required in those instances.
The following data is necessary to ensure reasonable results:
Additional information may be needed at some sites such as:
- intersection turning movement counts or projections
- intersection geometrics
- lane assignments
- special operating characteristics
The procedures are based on typically occurring situations subject to modification based on the data described above. Further adjustments are possible based on observed impacts of factors such as frequent bus stops, very large truck volumes, bikeway influences and so forth.
- pedestrian timing
- typical queue length
- lengths of turn lanes
- offsets or clearance times from adjacent signals
- Enter the available data on the worksheet.
- Utilizing the appropriate Lane Use Factors determine the critical lane volume which controls the green time required for each signal phase.
- For a two-phase signal operation the opposing lefts are added to the factored thru/right lane volume. The assumption is made that although they move on the same phase as thru traffic these left turns required green time within the phase. Two-phase signals will be assumed except where existing phasing is different or double left/right turn lanes are being provided. An additional phase or phases must be included in the latter cases.
- Each major movement occurring during the signal phase is calculated separately with the larger sum being noted as critical in controlling the time required for the phase.
- Where left turn phasing includes no overlaps, the left turn and thru phases must be calculated separately.
- Where left turn phasing includes overlaps with the thru movements the procedures will be the same as described above for each phase.
EXCLUSIVE RIGHT TURN LANES
Where the right lane is devoted to the exclusive use of right turn vehicles, a maximum lane volume should be computed separately for through movements and right turn movements. If a right turn phase overlap is provided with a left turn phase on the cross street, subtract the overlapping left turn volume from the right turn volume. The highest of the through or right turn lane volumes should be added to the opposing left turn volume, except where significant right turns on red occur.
FREE RIGHT TURNS
A free right turn is one which is not controlled by the traffic signal or stop sign. Normally the movement is isolated by a channelizing island and controlled by a yield sign. If the right turn movement is serviced by an exclusive right turn lane of sufficient length that right turning vehicles are not part of the queue of thru vehicles, the right turning volumes can be excluded from the critical lane analysis. Knowledge of the intersection can be used to combine a sufficient number (percent) of the right turns with the thru traffic to reflect actual peak hour operations. In the absence of such knowledge a queuing analysis could be done. As a rule-of-thumb 150 feet of exclusive right turn lane will permit excluding all right turns; less than 50 feet will require that all rights be included. Distances within that range suggest that a portion of the right turn volume be included.
RIGHT TURN ON RED
The number of vehicles which can take advantage of the RTOR feature vary greatly based on site and traffic characteristics. At higher volume intersections, as the Level-of-Service diminishes, few gaps are generally available for RTOR. Unless observations of the RTOR operations support excluding some right turns from the Critical Lane Analysis, this feature will normally not be considered.
NO SEPARATE LEFT TURN LANE
On multi-lane approaches with no separate left turn lane the impact of left turning traffic may be significant, especially on high volume roadways. Typically the left lane operates as a left turn lane with nearly all thru traffic avoiding this lane. Calculations for such an approach should be as follows:
The left turn volume will be adjusted using the PCE Factor (shared lane) of the 1985 HCM Pages 9-35. The opposing volume will be total thru's and rights. When the adjusted left turn volume is greater than the remaining volume being included in the analysis, the left most lane will be considered an exclusive left turn lane. The analysis will proceed with that assumption. For other cases the resulting left turn volume will be added to the rest of the approach volume and the appropriate lane use factor applied to the total.
ON LANE APPROACHES
Where a bypass of left turning vehicle is available the one lane approach should be treated as if there is a separate left turn lane. If no bypass area is available traffic on the one lane approach can proceed only when there is no vehicle waiting to turn left. This case should be analyzed using PCE (shared lane) equivalencies (1985 HCM pages 9-35) to modify the left turn volumes. The resulting total will be added to the rest of the approach volume and the appropriate lane use factor applied.
DOUBLE LEFT TURN LANES
Both the access to the double left turn lane and movements made immediately after the left turn will influence the distribution of traffic between the available lanes. Generally the distribution is less balanced than for thru lanes; thus the recommended lane use factor of 0.60. Variations observed at specific sites may suggest the use of different factor for this movement.
LANE USE FACTORS
Lane Use Factors are to be as follows:
No Lanes Factor
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.