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CHAPTER 10
GRADE SEPARATIONS AND INTERCHANGES
10.0 INTRODUCTION AND GENERAL TYPES OF INTERCHANGES
The ability to accommodate high volumes of traffic safely and efficiently through intersections depends largely on the arrangement that is provided for handling intersecting traffic. The greatest efficiency, safety, and capacity, and least amount of air pollution are attained when the intersecting through traffic lanes are grade separated. An interchange is a system of interconnecting roadways in conjunction with one or more grade separations that provide for the movement of traffic between two or more roadways or highways on different levels.
Interchange design is the most specialized and highly developed form of intersection design. The designer should be thoroughly familiar with the material in Chapter 9 before starting the design of an interchange. Relevant portions of the following material covered in Chapter 9 also apply to interchange design:
- general factors affecting design
- basic data required
- principles of channelization
- design procedure
- design standards
Material previously covered is not repeated. The discussion which follows covers modifications in the above-mentioned material and additional material pertaining exclusively to interchanges.
The economic effect on abutting properties resulting from the design of an intersection at-grade is usually confined to the area in the immediate vicinity of the intersection. An interchange or series of interchanges on a freeway or expressway through a community may affect large contiguous areas or even the entire community. For this reason, consideration should be given to an active public process to encourage context sensitive solutions. Interchanges must be located and designed to provide the most desirable overall plan of access, traffic service, and community development.
The type of grade separation and interchange, along with its design, is influenced by many factors such as highway classification, character and composition of traffic, design speed, and degree of access control. These controls plus signing requirements, economics, terrain, environment, and right-of-way are of great importance in designing facilities with adequate capacity to safely accommodate the traffic demands. Interchange types are characterized by the basic shapes of ramps, namely, diamond, loop, directional, "urban" and cloverleaf interchanges. Figures 10-1A, B, C, D and E illustrate these basic interchange types. These examples can further be classified as either local street interchanges or freeway-to-freeway interchanges.
Although each interchange presents an individual challenge, it must also be considered in conjunction with adjacent interchanges, driver expectancy, and at-grade intersections in the corridor as a whole. For further information, see Chapter 10 of the PGDHS (1). A more detailed description of all the basic types is found later in this chapter.
Figure 10-1A Interchange Types
Figure 10-1C Interchange Types
Figure 10-1D Interchange Types
reduction in crash frequency and severity, the operational efficiency for all traffic movements is also improved with interchanges.
- Site topography - The site topography and the grades of the intersecting roadways are important to determine interchange type and location. The right-of -way required for an interchange is dependent largely on the type of highway, topography, and the overall type of interchange.
- Road-user benefits
- Traffic volume warrant - Except on freeways, interchanges usually are provided only where crossing and turning traffic cannot readily be accommodated on a less costly at-grade intersection.
- Transit
- Functional classification of the road
10.2 ADAPTABILITY OF HIGHWAY GRADE SEPARATIONS AND
INTER-CHANGES
The three types of intersections are:
- at-grade intersections
- highway grade separations without ramps
- interchanges
Factors that would determine the need for an interchange and its type:
- Traffic and Operation
- Site Conditions
- Type of Highway - The hazard from stopping and direct turns at an intersection increases with the design speed so that high-design-speed highways warrant interchange treatment earlier than low-design-speed roads with similar traffic volumes.
- Intersecting Facility - The extent or degree to which local service must be maintained or provided also is of concern in the selection of the type of intersection. Local service can be provided readily on certain types of at-grade intersections, whereas considerable additional facilities may be necessary on some types of interchanges.
- Safety
- Stage Development - Where the ultimate development consists of a single grade-separation structure, stage construction may not be economical unless provisions are made in the original design for a future stage of construction. Ramps, however, are well adapted to stage development.
- Economics - Initial cost needs to be considered. The interchange is the costliest type of intersection because of the cost of the structure, ramps, through roadways, grading and landscaping of large areas.
- Maintenance costs may be a factor in the type of intersection. Interchanges have large pavement and variable slope areas, the maintenance of which, together with that of the structure, signs, and landscaping, exceeds that of an at-grade intersection.
In a complete analysis of the adaptability of interchanges, it is necessary to compare vehicular operating costs of all traffic with those for other intersections.
10.3 GRADE SEPARATION STRUCTURES
In any single separation structure, care should be exercised in maintaining a constant clear roadway width and a uniform protective railing or parapet.
The type of structure best suited to grade separations is one that gives drivers little sense of restriction. Where drivers take practically no notice of a structure over which they are crossing, sudden and erratic changes in speed and/or direction are unlikely. On the other hand, it is virtually impossible not to notice a structure overpassing the roadway being used. For this reason, every effort should be made to design the structure so that it fits the environment in a pleasing and functional manner without drawing unnecessary or distracting attention.
A detailed study should be made at each proposed highway grade separation to determine whether the main road should be carried over or under the structure. Often the choice is dictated by features such as cost, environmental impacts, topography, or highway classification. It may be necessary to make several nearly complete preliminary layout plans before a decision regarding the most desirable general layout plan can be reached.
As a rule, a design that best fits the existing topography is the most economical to construct and maintain, and this factor becomes the first consideration in design.
The clear width on bridges should be as wide as the approach pavement including shoulders, in order to give the driver a secure feeling. When the full approach roadway is continued across the structure, the parapet rail, both left and right, should align with the guardrail on the approach roadway.
Minimum lateral clearances at underpasses and retaining walls should include any provisions for the dynamic lateral deflection that the guardrail may require.
Additional information on vertical clearances is in section 3.3.
For more information on grade separation structures, see Chapter 10 of the PGDHS (1).
10.4 INTERCHANGES
10.4.1 General
There are several basic interchange forms or geometric patterns of ramps for turning movements at a grade separation. Their application at a particular site is determined by the number of intersection legs, the expected volumes of through and turning movements, topography, culture, design controls, proper signing, and the designer's judgment.
The design and selection of an interchange type are influenced by many factors as described elsewhere in this chapter. Even though interchanges are, of necessity, designed to fit specific conditions and controls, the pattern of interchange ramps along a freeway should follow some degree of consistency. From the standpoint of driver expectancy, all interchanges should have one point of exit located in advance of the crossroad wherever practical. It is desirable to rearrange portions of the local street system in conjunction with freeway construction in order to achieve an effective overall plan of traffic service and community development.
Data relative to community service (community access needs), traffic (projected traffic volumes), physical (topographic), environmental (NEPA considerations), economic factors (potential right- of-way acquisition), and potential area development which may affect design, should be obtained prior to interchange design (context sensitive solutions). Specifically, the following information should be available:
- The location and standards (types) of existing and proposed local streets and highway development including types of traffic (access) control.
- Present and potential traffic circulation over the affected local roads or streets.
- Existing and proposed land use including such developments as shopping centers, recreational facilities, housing developments, schools, churches, hospitals, and other institutions.
- A traffic flow diagram (a schematic interchange layout) showing annual average daily traffic and design hourly volumes, as well as time of day (a.m. or p.m.), anticipated on the freeway ramps and any affected local roads or streets.
- The relationship with (distances to and from) adjacent interchanges.
- The location of major utilities and multi-modal facilities (e.g., railroads, transit, airports).
10.5.1 Determination of Interchange Configuration
The need to use interchanges may occur in the design of roadways of all functional classifications, as discussed under section 10.1.
In rural areas, the problem of interchange-type selection is solved on the basis of service demand. The predominant rural interchange type in use in Colorado is the diamond interchange.
A combination of directional, semi-directional, and loop ramps may be appropriate where turning volumes are high for some movements and low for others. When loop ramps are used in combination with direct and semi-direct ramp designs, it is desirable that the loops be arranged so that weaving sections will not be created.
A cloverleaf interchange is the minimum design that can be used at the intersection of two fully controlled access facilities or where left turns at-grade are prohibited. A cloverleaf interchange is adaptable in a rural environment where right-of-way is not prohibitive and weaving is minimal. In the decision process to use cloverleaf interchanges, careful attention should be given to the potential improvement in operational quality that would be realized if the design included collector-distributor roads on the major roadway.
Simple diamond interchanges are the most common type of interchange for the intersection of a major roadway with a minor facility. The capacity of a diamond interchange is limited by the capacity of the at-grade terminals of the ramps at the cross road. High through and turning volumes could preclude the use of a simple diamond unless signalization is used.
Diverging diamond interchanges (DDI) are a variation of conventional diamond interchanges. The DDI uses directional crossover intersections to shift traffic on the cross street to the left-hand side between the ramp terminals within the interchange. Crossing the through movements to the opposite side replaces left-turn conflicts with same-direction merge/diverge movements and eliminates the need for exclusive left-turn signal phases to and from the ramp terminals. All connections from the ramps to and from the cross street are joined outside of the cross-over intersections, and these connections can be controlled by two-phase signals, have stop or yield
control, or can be free flowing. In addition to the added safety benefits, DDI’s typically have higher left-turn volume capacity and improved operations compared to conventional signalized diamond interchanges due to shorter cycle lengths, reduced time lost per cycle phase, reduced stops and delay, and shorter queue lengths.
Single-point urban interchanges (SPUI) is an interchange configuration that has all four turning movements controlled by a single traffic signal and the opposing left-turns operate to the left of each other, so their paths do not intersect. As a result, a major source of traffic conflict is eliminated, increasing the overall intersection efficiency and reducing the traffic signal phasing needed from four-phase to three-phase operation.
Partial cloverleaf designs may be appropriate where rights-of -way are not available in one or two quadrants or where one or two movements in the interchange are disproportionate to the others, especially when they require left turns across traffic. In the latter case, loop ramps may be utilized to accommodate the heavy left-turn volume.
Interchanges in rural areas are widely spaced and can be designed on an individual basis without any appreciable effect from other interchanges within the system.
The final configuration of an interchange may be determined by the need for route continuity, uniformity of exit patterns, single exits in advance of the separation structure, elimination of weaving on the main facility, signing potential, and available right-of-way.
Interchange-type determination in an urban environment requires considerable analysis of regional conditions so that the most practical interchange configurations can be developed.
10.5.2 Approaches to the Structure
See the PGDHS (1) Chapter 10.
10.5.2.1 Alignment, Profile, and Cross Section
Traffic passing through an interchange should be afforded the same degree of utility and safety as that given on the approaching highways. The design elements in the intersection area, therefore, should be consistent with those on the approaching highways, even though this may be difficult to attain. Preferably, the geometric design at the highway grade separation should be better than that for the approaching highways to counterbalance any possible sense of restriction caused by the structure. When it is practical to design only one of the intersecting roadways on a tangent with flat grades, it should be the major highway.
The general controls for horizontal and vertical alignment and their combination, as stated in Chapter 3, should be adhered to closely. Particular attention should be given to providing decision sight distance in situations where drivers must make complex or instantaneous decisions within interchanges.
The longitudinal distance needed for adequate design of a grade separation depends on the design speed, the roadway gradient, and the amount of rise or fall needed to achieve the separation. The amount of rise or fall needed will depend on the amount of vertical clearance needed in addition to the structure depth. The approximate distance needed to achieve a grade separation (assuming
Figure 10-2 [Figure 10-50 of the PGDHS (1)] Typical Examples of Lane Balance
Figure 10-3 [Figure 10-51 of the PGDHS (1)] Coordination of Lane Balance and Basic Number of Lanes
Figure 10-4A Types of Weaving Sections
Figure 10-4B Types of Weaving Sections
