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  Capacity Analysis of Pedestrian and Bicycle Facilities:
Recommended Procedures for the "Signalized Intersections" Chapter of the Highway Capacity Manual


4 PROPOSED REVISIONS TO HCM CHAPTER 9 PROCEDURES

4.1 Overview of Recommended Procedure for Determining fLpb and fpb

      This section summarizes the recommended procedure for calculating the value of an adjustment factor that describes the effect of pedestrians and bicycles on lane group saturation flow. For left turns, the adjustment is termed fLpb; for right turns, the adjustment is termed fRpb. The procedure consists of four basic parts that correspond to the four phases of the data reduction methodology described earlier. They are:

Part 1: Determine average pedestrian occupancy, OCCpedg, during the entire pedestrian green;

Part 2: Find relevant conflict zone occupancy, OCCr , by adjusting OCCpedgas needed for opposing traffic (left turns) or conflicting bicycles (right turns);

Part 3: Compute permitted phase saturation flow adjustment just for turning vehicles due to pedestrian and bicycle interference, ApbT; and

Part 4: Determine saturation flow adjustment factor for the lane group fLpb for left turns and fRpbt. for right turns.

      Table 2 contains two groups of parameters that comprise all of the input requirements needed to determine fLpb and fRpb. The first group lists several qualitative intersection parameters, while a second group contains quantitative parameters needed to complete the procedure. Within each group, the table lists the parameters in the order the procedure first needs them. While one will need between 9 and 13 input parameters, depending on the situation, the proposed procedure does not require any additional field data collection. In other words, the procedure requires no (zero) new input parameters beyond those needed for the current HCM. The following paragraphs provide an overview of each of the four parts. To aid the user, Figure 7 provides a flowchart, which serves as a visual outline to the procedure. In addition, Table 3 provides a list of symbols used in the computation of fLpb and fRpb.

TABLE 2
Input Requirements for Determination of fRpb and fLpb

Qualitative Parameter

Turn direction (left or right)
Street type (one-way or two-way)
Turn lane type (exclusive, shared, or single)
Signal phasing type (protected, permitted, or protected-permitted)

 

Quantitative Parameter (also consult Figure 2) Symbol
Cycle Length (s) C
Extent of Opposing Vehicle Queue (s)a gq
Opposing Flow Rate After Queue Clears (veh/h) a vo
Effective Number of Turning Lanes Nturn
Effective Number of Departure Lanes Ndep
Proportion of Left- or Right-turns in Lane Groupb PLT ; PRT
Proportion of Left- or Right-turns using Protected Phasec PLTA ; PRTA
Pedestrian Volume (peds/h or peds/h ped-green) d Vped or Vpedg
Bicycle Volume (bikes/h or bikes/h green) e Vbike or Vbikeg
Effective Green (for vehicles or bicycles, s) f g
Ped Green Time (Walk + Flashing Don=t Walk), s g gp
anecessary only for left turns from a two-way street; see 1994 HCM, page 9-20
bnecessary only for right turns from a single lane approach or for a shared turning lane
cnecessary only if protected plus permitted phasing
dignore those pedestrians who cross against the green (i.e., noncompliant pedestrians)
enecessary only for right turns impeded by bicycles
fultimately needed in all cases to compute lane group capacity; however, only necessary
at this point in the procedure for right turns impeded by bicycles
gif no pedestrian signal, use g as a proxy for gp; if numerous pedestrians crossing the
street after the conclusion of the flashing DON=T WALK conflict with turning vehicles,
extend the effective pedestrian green time accordingly


Figure 7
Outline of computational precedure for fRpb and fLpb



TABLE 3    List of symbols used in determination of fRpb and fLpb
Cycle Length (s) C
Pedestrian Volume (pedestrians/h) Vped
Pedestrian Flow Rate (pedestrians/h of green) Vpedg

Pedestrian Green Time (Walk + Flashing Don=t Walk), s

gp
Average Pedestrian Occupancy During the Effective Pedestrian Green Time OCCpedg
Bicycle Volume (bicycles per h) Vbike
Effective Green (for vehicles or bicycles/s) g
Bicycle Flow Rate (bicycles/h of green) Vbikeg
Average Bicycle Occupancy During the Effective Green Time OCCbikeg
Extent of Opposing Vehicle Queue (s) gq
Opposing Flow Rate After Queue Clears (vehicles/h) vo
Average Pedestrian Occupancy After the Opposing Queue Clears OCCpedu
Relevant Conflict Zone Occupancy From the Driver=s Perspective OCCr
Effective Number of Turning Lanes Nturn
Effective Number of Receiving Lanes Nrec
Permitted Phase Pedestrian-Bicycle Adjustment for Turning Vehicles ApbT
Proportion of Left or Right turns in Lane Group PLT ; PRT
Proportion of Left or Right turns Using Protected Phase PLTA ; PRTA
Pedestrian-Bicycle Adjustment Factor for Right Turns fRpb
Pedestrian Adjustment Factor for Left Turns fLpb


      The first part of the procedure determines the average occupancy of the conflict zone over the entire pedestrian green phase, OCCpedg. Practitioners can utilize existing counts by converting them to an hourly flow rate using the equations listed. Alternatively, if one counted pedestrians for an entire hour of pedestrian green time for a movement, the user could then enter the resulting count as the pedestrian volume/h green (Vpedg) without conversion. If possible, data collectors should only count those pedestrians who conflict with turning vehicles.

Figure 8
Pedestrians causing substantail delay to an "unopposed" left turn in Portland, Oregon
      The second part determines the relevant occupancy of the conflict zone from the perspective of the turning driver, OCCr. Follow the appropriate group of steps depending on the potential for interference by either opposing vehicles (left turns) or bicycles (right turns), if any. Of course, even an Aunopposed@ left turn can still experience a substantial reduction in turning capacity (Figure 8). In addition, based on field observations at California, Oregon, and Florida, if bicycle traffic weaves with right-turning traffic in advance of the stop-bar, the interaction between bicycles and right-turning vehicles is completely independent of the interaction with pedestrians, and one should ignore the bicycle volume when analyzing the signalized intersection. In other words, while weaving between bicycles and right turns may take place some distance upstream from the intersection, the interaction between pedestrians and right turns will occur at the intersection itself.

      The third part determines the adjustment to turning vehicle saturation flow during the permitted phase due to pedestrian or bicycle interference, Apbt. Use the effective (i.e., Aas actually used@) number of turning lanes (Nturn) and receiving lanes (Nrec), which may or may not match those suggested by traffic control devices. For example, vehicles may consistently turn from an outer lane illegally, or double-parked vehicles may block a turn or receiving lane.

      The fourth part determines the actual saturation flow adjustment factor, fRpb or fLpb. This factor represents the adjustment to saturation flow for a lane group containing turning vehicles subject to pedestrian and/or bicycle interference. One can Agrossly estimate@ the proportion of right turns using the protected phase (PRTA ) as the proportion of the green phase that is protected, as suggested in the HCM on page 9-18 (TRB, 1994). Also, one can Agrossly estimate@ the proportion of left turns using the protected phase (PLTA ) as equal to (1- permitted phase fLT) / 0.95.
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