Misión

Procedures

In the design use of the procedures in this manual, it is necessary to consider the kinds of information that generally would be avail- able and what results are desired. Capacity analysis is only one of several inputs into the design process. Others include geometric standards, safety standards, standards for signing, and so on.

Capacity analysis procedures are used primarily in the design of cross-sectional elements (number of lanes, lane widths, shoul- ders) and in the selection of lane configurations for individual freeway elements. In general, the following information is required for a design analysis:

T Horizontal and vertical alignments.

T Approximate location of ramps and interchanges. T Forecast demand volumes.

T Forecast demand characteristics, such as, to name a few, the

percentages of trucks, buses, and recreational vehicles in the traffic stream, and peak-hour factor (PHF).

The principal problem in coordinating the overall design analy- sis of a freeway facility is the segmenting of the freeway into component parts for individual consideration using the methods of Chapters 3, 4, and 5. In general, the following guidelines may be used:

1. Each section of freeway between ramps or major junctions should be considered to be a separate ‘‘basic freeway segment.’’ 2. Within these basic freeway segments, any grade of more than 1_⁄

4mi (for grades≥3 percent) or1⁄2mi (for grades <3 percent) must be considered a separate basic freeway segment. Any sharp change in terrain, such as from level to rolling terrain, would also necessi- tate the division of a single segment into separate subsegments. Long basic segments with no single grade of significance may be considered extended segments of level, rolling, or mountainous terrain, as defined in Chapter 3. Downgrade segments would nor- mally be considered to be level terrain unless local data allow for more specific treatment (see Chapter 3).

3. Each ramp junction should be considered once in combina- tion with the adjacent downstream ramp and once in conjunction with the adjacent upstream ramp. Ramps that are clearly part of a weaving section would not be analyzed using ramp procedures but would be treated in Step 4.

4. Potential weaving and multiple weaving areas should be in- vestigated as such. The term ‘‘potential’’ is used because some

present and significantly affect operations. Because of the many complexities of freeway system operations, these procedures tend to be more approximate and less precise than those applied to specific freeway subsections. They nevertheless provide a basis for insight and understanding of system effects.

segments may turn out to be either weaving areas or ramp combina- tions, depending on the final configuration adopted.

In application, these guidelines lead to fairly straightforward computations in the following sequence:

1. Establish design level of service, demand volume and traffic characteristics, horizontal and vertical alignments, and approxi- mate ramp locations.

2. Determine the basic number of lanes required for each of the basic freeway segments identified as previously noted, using the procedures detailed in Chapter 3. The basic number of lanes for each ramp may be determined using techniques described in Chapter 5.

3. The results of Step 2 will suggest probable configurations for ramp junctions and potential weaving areas. Analyze each ramp junction from three points of view: (a) as an isolated ramp, (b) in combination with the adjacent downstream ramp, and (c) in combination with the adjacent upstream ramp using the procedures in Chapter 5. Usually, one or two of these aspects will be invali- dated by those procedures, but in other cases, there will be more than one valid analysis. In such cases, the analysis indicating the poorest operations or level of service is taken as the controlling solution.

4. Analyze the weaving areas using the procedures in Chapter 4 to determine likely operating conditions. Note that in design, the case of an on-ramp followed by an off-ramp must be regarded as both a potential weaving section with an auxiliary lane and a ramp combination without an auxiliary.

5. If the results of Steps 3 and 4 are unsatisfactory, consider

T Altering the number and/or location of ramps (which may

affect demand distribution).

T Changing the design of ramps and/or mainline segments deter-

mined in Step 2 to create new configurations.

T Changing the design of major interchanges to achieve differ-

ent configurations, reduce weaving, and so forth. Repeat Steps 2 through 4.

Sample Calculation

The design indicated in Figure 6-1 illustrates the foregoing pro- cedures. Note that the given demand volumes are already ex- pressed as peak rates of flow in passenger cars per hour.

Figure 6-1. Sample design problem.

Step 1—Establish Demand, Alignment, and Ramp Location

Figure 6-1 shows demand, alignment, and ramp locations for the sample problem.

Step 2—Determine Basic Number of Lanes for Open Freeway Segments and Ramps

The demand on each open freeway segment is shown in Figure 6-1. Using the criteria in Table 3-1 directly for LOS B, the number of lanes in each segment may be found. Because of design deci- sions, 12-ft lanes, adequate lateral clearance, 70-mph free-flow speed (SFF) on the freeway mainline, and 45-mph free-flow speed

(SFR= 45 mph) on the ramps are to be provided.

No. of Lanes

Segment Flow Rate Required

1 2,900 3

2 3,400 3 to 4

3 4,000 4

4 3,600 3 to 4

5 3,300 3

Table 5-6 may be used to estimate the number of lanes required for each of the ramps. Using the 45-mph free-flow speed criterion, all of the ramps in Figure 6-1 are single-lane ramps. All accelera- tion and deceleration lanes are 250 ft long (LAand LD= 250 ft).

On the basis of these results, the design in Figure 6-2 is most likely to be appropriate. Note that because there is an auxiliary lane between Ramps B and C, Segments 2, 3, and 4 make up a multiple weaving area in this design.

Step 3—Analyze Ramp Junctions

Given that Ramps B and C are definitely part of a weaving section for the trial design in Figure 6-2, the following ramp combi- nations remain to be analyzed with ramp procedures:

T Ramp A, isolated or with a downstream on-ramp (B), T Ramp D, isolated or with an upstream off-ramp (C).

Ramps A and D could conceivably be considered both isolated ramps with a simple weaving section in Segment 3 and part of a multiple weaving configuration with Segment 3. Both cases would be analyzed.

Ramp A. According to Figure 5-3 for on-ramps, Equation 1 applies. Substituting into the equation gives the following results: V12= 1,695 pcph VFO= 3,400 pcph VR12= 2,195 pcph

Since VFO< 6,900 and VR12< 4,600, traffic flow is operating below

capacity, and density and speed can be determined from Tables 5-3 and 5-4, respectively. This gives DR= 21 pc/mi/ln and SR=

61 mph. From Table 5-2, LOS = C.

Ramp D. According to Figure 5-4 for off-ramps, Equation 7 applies. Substituting into the equation yields the following results:

V12= 2,465 pcph VFO+ VR= 3,600 pcph

Since VFO+ VR< 6,900 and V12< 4,400, traffic flow is operating below capacity, and density and speed can be determined from Tables 5-3 and 5-4, respectively. This gives DR= 23 pc/mi/ln and

SR= 61 mph. From Table 5-2, LOS = C.

Ramps B and C should not be considered part of the ramp configuration because the trial design in Figure 6-2 shows them to be in a weaving configuration; as such, they are analyzed in Step 4.

Step 4—Analyze Potential Weaving Areas

Segments 2 and 3 should be considered a multiple weave. For the purposes of this analysis, all off-ramp vehicles at C will be assumed to originate from the freeway mainline, a worst-case as- sumption. Figure 6-3 depicts the resulting flows and weaving diagrams.

Segment 2. Because one of the Segment 2 weaving movements is made with no lane change and another with one lane change, this is a Type B section. For Segment 2,

VR = 900/3,400 = 0.26 R = 400/900 = 0.44

Figure 6-2. Likely design for sample problem.

Figure 6-3. Consideration of multiple weave.

Applying Equations 4-2 and 4-3, the speed of weaving and non- weaving vehicles is computed:

Swor Snw= 15 +

SFF− 10

1 + a(1 + VR)b_(v/N)c_/Ld

where, from Table 4-3 for unconstrained Type B sections: Constant SwComputation SnwComputation

a 0.100 0.020 b 1.20 2.00 c 0.77 1.42 d 0.50 0.95 and SFF= 70 mph, v = 3,400 pcph, N = 3 lanes, and L = 2,000 ft.

This results in the following estimates of speed for unconstrained operation:

Sw= 51.0 mph

Snw= 54.9 mph

To determine whether operations are actually unconstrained, the number of weaving lanes used is now computed by using the equation given in Table 4-4:

Nw= N[0.085 + 0.703VR + (234.8/L) − 0.018(Snw− Sw)]

where Snwand Sware as computed above. Substituting the appro-

priate values,

Nw= 0.95 lane

Nnw= 3 − 0.95 = 2.05 lanes

Because Nw is less than the maximum value of 3.50 lanes for

Type B sections (Table 4-4), the section is unconstrained, and the original estimates of weaving and nonweaving speeds are taken to be correct. On the basis of the calculated speeds and effective lanes for weaving and nonweaving traffic, the corresponding densi- ties are computed as follows:

Dw= Vw NwSw = 900 0.95 × 51= 18.6 pc/mi/ln Dnw= Vnw NnwSnw = 2,500 2.05 × 54.9= 22.2 pc/mi/ln

According to Table 4-6, weaving traffic operates at LOS B, whereas nonweaving traffic operates at LOS C. A joint measure for the entire traffic stream can be estimated using the average overall speed (weighted by volume), S, where

S =(900 × 51) + (2,500 × 54.9)

and the corresponding density, D = 3,400

3 × 53.9= 21 pc/mi/ln yielding an overall LOS C for the weaving section.

Segment 3. This segment should be considered a Type A weav- ing area because it has an auxiliary lane, as shown in Figure 6-3, and all weaving vehicles make at least one lane change. Note that consideration of Segment 3 as a multiple weave is the same as considering it as a simple weaving section. For Segment 3,

VR = 1,000/4,000 = 0.25 R = 400/1,000 = 0.40

From Table 4-3, for unconstrained Type A weaving areas: Constant SwComputation SnwComputation

a 0.226 0.020 b 2.20 4.00 c 1.00 1.30 d 0.90 1.00 and SFF= 70 mph, v = 4,000 pcph, N = 4 lanes, and L = 1,500 ft. Then Sw= 54.7 mph Snw= 62.7 mph

From Table 4-4, the minimum number of weaving lanes needed to support unconstrained operation is

Nw= 2.19NVR0.571LH0.234/Sw0.438

Nw= 1.30 lanes

Nnw= 4 − 1.30 = 2.70 lanes

Because Nwis less than the maximum value of 1.4 lanes given in

Table 4-4, the operation is unconstrained, and the computed speeds are correct. Using the same process as that for Segment 2, the following speeds and densities are calculated for Segment 3:

Dw= 1,000 1.3 × 54.7= 14.1 pc/mi/ln Dnw= 3,000 2.7 × 62.7= 17.7 pc/mi/ln S =(1,000 × 54.7) + (3,000 + 62.7) 1,000 + 3,000 = 59.5 mph D = 4,000 4 × 59.5= 16.8 pc/mi/ln

A review of the LOS criteria in Table 4-6 indicates that all elements of Segment 3 operate at LOS B.

Segments 3 and 4. These segments should now be considered a multiple weaving area, as shown in Figure 6-4. Again, it will be

Figure 6-4. Consideration of multiple weave.

assumed that no on-ramp vehicles at B leave that freeway at C or D (a worst-case assumption).

Segment 3, in this case, remains the same as previously, so no additional analysis is required.

Segment 4, however, should be analyzed as a Type B weaving section, because one weaving movement is made with no lane change and the other requires only one lane change. For Segment 4:

VR = 900/3,600 = 0.25 R = 300/900 = 0.33

Constant SwComputation SnwComputation

a 0.100 0.020 b 1.20 2.00 c 0.77 1.42 d 0.50 0.95 and SFF= 70 mph, v = 3,600 pcph, N = 3 lanes, and L = 2,500 ft. Then Sw= 52.2 mph Snw= 56.8 mph

From Table 4-4, the number of weaving lanes required for un- constrained operation is

Nw= N[0.085 + 0.703VR + (234.7/L) − 0.018(Snw− Sw)]

Nw= 0.81 lane

Nnw= 3 − 0.81 = 2.19 lanes

Because Nw is less than the maximum allowable value of 3.50

lanes (Table 4-4), the operation is unconstrained, and the computed speeds are correct. Using the same process as that for Segments 2 and 3, the following densities and speed are calculated for Seg- ment 4: Dw= 900 0.81 × 52.2= 21.3 pc/mi/ln Dnw= 2,700 2.2 × 56.8= 21.7 pc/mi/ln S =(900 × 52.2) + (2,700 + 56.8) 900 + 2,700 = 55.7 mph D = 3,600 3 × 55.7= 21.5 pc/mi/ln

A review of the LOS criteria in Table 4-6 indicates that all elements on Segment 4 operate at LOS C.

Given that all of the weaving areas and ramp junctions meet the minimum LOS criteria established for the design, the trial design of Figure 6-2 would appear to be acceptable for implementation.