Procedural steps for the analysis of simple weaving areas are given below. Computations are performed in the operational anal- ysis mode; that is, a known or projected situation is analyzed for the probable level of service. All roadway and traffic conditions must be specified, including weaving length, type of configuration, number of lanes, lane widths, terrain or grade, weaving and non- weaving flow rates by movement, peak-hour factor, and traffic composition.
Weaving analysis is made easier through the use of a weaving diagram, which is a schematic drawing showing weaving and nonweaving flows in a weaving area. Figure 4-6 shows such a diagram. Note that the weaving diagram depicts actual flows in
Table 4-6. LOS Criteria for Weaving Areas maximum density (pc/mi/ln)
level of freeway multilane and c-d
service weaving area weaving areas
A 10 12 B 20 24 C 28 32 D 35 36 E ≤43 ≤40 F >43 >40
where S is the average (space mean) speed of all vehicles in the weaving section in miles per hour, and all other variables are as previously defined. The density is then found:
D =v/N
S (4-5)
where D is the density in passenger cars per mile per lane. Table 4-6, contains LOS criteria based on density in the weaving area. Note that criteria are shown for freeways as well as for multilane highways and collector-distributor (C-D) roadways. The procedures in this chapter can be applied to weaving sections on multilane highways by using an appropriate free-flow speed in the prediction of nonweaving and weaving vehicle speeds. Chapter 7 contains procedures for the estimation of free-flow speed on a multilane highway if field measurements are not available. Multi- lane criteria may be cautiously applied to C-D roadway weaving areas, but it is recommended that free-flow speed be measured or roughly estimated from design speed or speed limit information.
In general, these criteria allow for slightly higher densities at any given LOS threshold than on a comparable basic freeway or multilane highway section. This follows the philosophy that drivers expect higher densities in weaving areas relative to those on basic freeway or multilane highway segments. The LOS E- F boundary does not apply this approach. Rather, it is thought that breakdown will occur at slightly lower densities than on basic sections because of the additional turbulence resulting from weaving movements.
a straight-line form. The relative placement of entry and exit points (A, B, C, D) in the diagram matches the actual site to ensure proper placement of weaving and nonweaving flows relative to each other. Flows on the weaving diagram should represent flow rates for the peak 15 min under ideal conditions, expressed in passenger cars per hour. It is also convenient to use the weaving diagram as a guide in computing the parameters used during an analysis.
The level of service in an existing or projected weaving area is evaluated using the following computational steps.
Step 1—Establish Roadway and Traffic Conditions
All existing or projected roadway and traffic conditions must be specified. Roadway conditions include the length, number of
Figure 4-6. Construction and use of weaving diagrams.
lanes, and type of configuration for the weaving area under study. Table 4-1, should be consulted in assigning the type of configura- tion. Other roadway features of importance are lane widths and the general terrain or grade conditions for the section.
Traffic conditions include the distribution of vehicle types in the traffic stream, as well as the peak-hour factor, or peak-hour factors where the component flows have differing peaking characteristics. Because the weaving area should be analyzed on the basis of peak flow rates for a 15-min interval within the hour of interest, hourly volumes must be adjusted by dividing by the peak-hour factor. Such a conversion, however, ignores the fact that the four component flows in a weaving area may not all peak during the same interval. Where possible, weaving flows should be observed and recorded for 15-min intervals so that critical periods may be identified for analysis. Where hourly volumes are available or projected, it will be assumed that all component flows peak simul- taneously—a conservative procedure. The predicted speeds of weaving and nonweaving vehicles will be lower than those actually occurring in such cases. It should also be noted that the component movements in a weaving area may not have the same peak-hour factor. Where possible, each flow and its peaking characteristics should be considered separately.
Step 2—Convert All Traffic Volumes to Peak Flow Rates Under Ideal Conditions
Because all of the speed and lane-use algorithms presented ear- lier are based on peak flow rates under ideal conditions, expressed in passenger cars per hour, all component flows must be converted to this basis:
v = V
PHF × fHV× fw× fp
(4-6) where
v = flow rate for peak 15 min under ideal conditions (pcph); V = hourly volume under prevailing condition (veh/hr); PHF = peak-hour factor;
fHV= heavy-vehicle adjustment factor, determined using the
procedures in Chapter 3 or 7; and
fp= driver population adjustment factor, determined using the
procedures in Chapter 3.
Step 3—Construct Weaving Diagram
A weaving diagram of the type illustrated in Figure 4-6 is now constructed, with all flows indicated as peak flow rates under ideal conditions in passenger cars per hour. Critical analysis variables are identified and computed as shown in Figure 4-6.
Step 4—Compute Unconstrained Weaving and Nonweaving Speeds
Using weaving intensity factors for the appropriate configuration from Table 4-3, compute the average (space mean) speed for weav- ing and nonweaving vehicles. Unconstrained operation is assumed for this step.
Step 5—Check for Constrained Operation
Using the speeds computed in Step 4, estimate the number of lanes needed by weaving vehicles to achieve unconstrained opera- tion using the equations in Table 4-4. Compare the computed value of Nwwith the tabulated value of Nw(max) to determine whether
operation is constrained or unconstrained.
If Nw ≤ Nw (max), the operation is unconstrained, and the
speeds computed in Step 4 are accurate. If Nw > Nw (max), the
operation is constrained. Values of Swand Snwmust be recomputed
using Equation 4-3 and the constrained weaving intensity factor for the appropriate configuration given in Table 4-3.
Step 6—Compute Average (Space Mean) Speed and Density of All Vehicles in Weaving Area
Use Equation 4-4 to compute the average (space mean) speed of all vehicles in the weaving section. The result may be used in Equation 4-5 to compute the density in weaving section. Then
S =vw+ vnw vw Sw +vnw Snw D =v/N S
Step 7—Check Weaving Area Limitations
Table 4-5 should be consulted to ensure that none of the limita- tions specified are exceeded. Where one or more of these limits are exceeded, consult the Methodology section of this chapter for the appropriate interpretation.
Care should be taken in applying the limiting values given in Table 4-5. Where the weaving capacity is exceeded, it is likely that breakdowns will occur and that LOS F will prevail, at least for weaving vehicles. Where limitations on VR or R are exceeded, breakdowns need not occur, but speeds would be lower than those anticipated by the equations of Table 4-3. Maximum lengths reflect the limits of the predictive equations. Lengths beyond the values shown may be analyzed as separate merge and diverge areas using the procedures in Chapter 5. It would not be expected that speeds within the section would be significantly lower than those for a basic freeway section serving the same volume.
Step 8—Determine Level of Service
The estimated value of density, D, in the weaving area is com- pared with the criteria in Table 4-6 to determine the prevailing level of service.