Posición y separación horizontal de las luces

The requirement that drivers execute lane changes to complete many weaving movements introduces a new geometric parameter for consideration—weaving length. The length of the weaving sec- tion constrains the time and space in which the driver must make all required lane changes. Thus, as the length of a weaving area decreases (all other factors being constant), the intensity of lane- changing, and the resulting level of turbulence, increases. The measurement of weaving area length is shown in Figure 4-2. Length is measured from the merge gore area at a point where the right edge of the freeway shoulder lane and the left edge of the merging lane(s) are 2 ft apart to a point at the diverge gore area where the two edges are 12 ft apart.

Procedures in this chapter generally apply to weaving sections of up to 2,500 ft in length. Weaving may exist in longer sections, but merging and diverging movements are often separated, with lane-changing tending to concentrate near merge and diverge gore areas. For longer sections, merge and diverge areas may be sepa- rately analyzed using the procedures of Chapter 5. Weaving turbu- lence may exist throughout a long section to some degree, but operations are approximately the same as those for a basic freeway section.

CONFIGURATION

Because lane-changing is the critical operational feature of weaving areas, another critical geometric characteristic can drasti- cally affect performance: configuration. Configuration refers to the relative placement and number of entry lanes and exit lanes for the section, and it can have a major impact on how much lane- changing must take place in the section.

The procedures in this chapter deal with three primary types of weaving configuration. These are referred to as Type A, Type B, and Type C sections, and are shown in Figures 4-3, 4-4, and 4-5, respectively. The types are defined in terms of the minimum num- ber of lane changes that must be made by weaving vehicles as they travel through the section.

Type A Weaving Areas

Type A weaving areas require that each weaving vehicle make one lane change in order to execute the desired movement. Figure 4-3 shows two examples of Type A weaving areas. In Figure 4-3(a),

Figure 4-3. Type A weaving areas: (a) ramp-weave/one-sided weave, and (b) major weave.

an on-ramp is followed by an off-ramp, with a continuous auxiliary lane between the ramps. All on-ramp vehicles must make a lane change out of the auxiliary lane into the shoulder lane of the freeway, and all off-ramp vehicles must make a lane change from the shoulder lane of the freeway to the auxiliary lane. Lane changes to and from the outer lanes of the freeway may also take place within the section, but these are not mandated or required by the weaving movement.

Sections formed by on-ramp/off-ramp sequences joined by con- tinuous auxiliary lanes are often referred to as ramp-weave sec- tions. They may also be referred to as one-sided weaving sections,

Figure 4-4. Type B weaving areas: (a) major weave with lane balance at exit gore, (b) major weave with merging at entrance gore, and (c) major weave with merging at entrance gore and lane balance at exit gore.

Figure 4-5. Type C weaving areas: (a) major weave without lane balance or merging, and (b) two-sided weave.

because all weaving movements take place on one side of the roadway. It should be noted that on-ramps followed by off-ramps that are not joined by a continuous auxiliary lane are not considered to be weaving areas. They are treated as separate merge and diverge areas and analyzed using the procedures of Chapter 5.

Figure 4-3(b) illustrates a major weaving section. Major weav- ing sections are characterized by three or more entry and exit roadways having multiple lanes. In Figure 4-3(b), two two-lane sections join to form a four-lane roadway, only to separate into two two-lane sections again at the diverge point. Note that all weaving vehicles must make at least one lane change, regardless of the direction in which they are weaving.

Figure 4-3(a) and 4-3(b) are similar in that each has a crown line, that is, a lane line that connects the nose of the entrance gore area to the nose of the exit gore area. The lane change that each weaving vehicle must make is across this crown line.

The two sections illustrated differ primarily in the impact of ramp geometrics on speed. For many ramp-weave sections, the design speed of ramps is significantly lower than that of the free- way. Thus, on- or off-ramp vehicles must accelerate or decelerate as they traverse the weaving section. For major weaving sections, the design of multilane entry and exit legs is more compatible with the design of the freeway mainline, and the impact of accelera- tion and deceleration in the section is minimal. It should be noted, however, that this difference is not reflected in the procedures in this chapter because of the relative scarcity of major weave sites with crown lines and the lack of data concerning operations in such sites.

Because weaving vehicles in a Type A weaving area must cross the crown line, weaving vehicles are usually confined to occupying the two lanes adjacent to the crown line while in the weaving section. Normally, some nonweaving vehicles will also remain in lanes adjacent to the crown line. Lanes adjacent to the crown line are, therefore, generally shared by weaving and nonweaving vehicles. One of the most significant effects of configuration on

operations is to limit the maximum number of lanes that weaving vehicles may occupy while traversing the section.

Type B Weaving Areas

All weaving areas classified as Type B may also be referred to as major weaving sections, because all involve multilane entry legs or exit legs or both. Two critical characteristics distinguish Type B weaving areas from all others:

1. One weaving movement may be accomplished without mak- ing any lane changes.

2. The other weaving movement requires at most one lane change.

Figure 4-4(a) and (b) show two such weaving areas. In both illustrations, movement B-C can be made without executing any lane changes, whereas movement A-D requires only one lane change. In Figure 4-4(a), this is accomplished by providing a diverging lane at the exit gore. From this lane, a vehicle may proceed on either exit leg without making a lane change. This type of design is also referred to as lane balanced, that is, the number of lanes leaving the diverge is one greater than the number of lanes approaching it. In Figure 4-4(b), a lane from Leg A is merged with a lane from Leg B at the entrance gore area.

Type B weaving areas are extremely efficient in carrying large weaving volumes, primarily because of the provision of a through lane for one of the weaving movements. Weaving maneuvers can be accomplished with a single lane change from the lane or lanes adjacent to this through lane. Thus, weaving vehicles can occupy a substantial number of lanes in the weaving section and are not as restricted in this regard as in Type A sections.

Figure 4-4(c) shows an unusual configuration in which both a merge of two lanes at the entrance gore and lane balance at the exit gore are provided. In this case, both weaving movements can be made without a lane change. Again, weaving movements can be made with a single lane change from the two lanes adjacent to the through lane. Such configurations are usually found on collec- tor-distributor roadways. Although some weaving movements are accomplished as a merge followed by a diverge, lane changes to and from lanes adjacent to the through lane yield real weaving activity, and these sections are analyzed as weaving areas.

Type C Weaving Areas

Type C weaving areas are similar to Type B sections in that one or more through lanes are provided for one of the weaving movements. The distinguishing feature between Type B and Type C sections is the number of lane changes required for the other weaving movement. A Type C weaving area is characterized as follows:

1. One weaving movement may be accomplished without mak- ing a lane change.

2. The other weaving movement requires two or more lane changes.

Figure 4-5 shows two Type C weaving areas. In Figure 4-5(a), movement B-C does not require lane-changing, whereas movement A-D requires two lane changes. This type of section is formed when there is neither a merging of lanes at the entrance gore nor

Table 4-1. Configuration Type Versus Minimum Number of Required Lane Changes

}

a

}

b

minimum number minimum number of req’d lane

of req’d lane changes for

changes for weaving mvt. b

weaving mvt. a 0 1 ≥2

0 Type B Type B Type C

1 Type B type A —

≥2 Type C — —

lane balance at the exit gore and no crown line exists. Although such a section is relatively efficient for weaving movements in the direction of the through lane, it cannot efficiently handle large weaving volumes in the other direction.

Figure 4-5(b) shows a two-sided weaving area. It is formed when a right-hand on-ramp is followed by a left-hand off-ramp or vice versa. In such cases, the through volume on the freeway is functionally a weaving movement. Ramp-to-ramp vehicles must cross all lanes of the freeway to execute their desired maneuver. Freeway lanes are, in effect, through weaving lanes. Ramp-to- ramp drivers must execute three lane changes in Figure 4-5(b). Although it is technically a Type C configuration, there is little information concerning the operation of such sections, and the methodology of this chapter is only a rough approximation of their characteristics. They should generally be avoided in cases where there is any significant ramp-to-ramp volume.

Determining Configuration Type

Figures 4-3, 4-4, and 4-5 show the three basic types of weaving area configuration. Weaving configuration is determined on the basis of the number of required lane changes that must be per- formed by the two weaving flows in the section. This determination ignores lane changes that are not necessary to the completion of a particular weaving movement. Table 4-1 identifies the configura- tion type on the basis of lane-changing characteristics.