234.2 Diamond Interchanges

Key Points

Figures

Geometric Layout of a Diamond Interchange

Ramp Layout Details of a Diamond Interchange

Deceleration Lane Length

Escape Lane

Ramp Nose Treatment and Pav't Width Transition

Green Book Exhibit 10-70, "Minimum Acceleration Lengths for Entrance Terminals"

Green Book Exhibit 10-71, "Speed Change Lane Adjustment Factors"

Green Book Exhibit 10-73, "Minimum Deceleration Lengths for Exit Terminals"

Green Book Exhibit 10-76, "Two Lane Exit Terminals"

Green Book Exhibit 10-77, "Typical Two-Lane Entrance Ramps"

234.2.1 Introduction

Advantages

• Most common interchange configuration; therefore, it meets driver expectation.
• Well suited for both rural and urban use.
• Adaptable in major-minor road crossings where grade crossings on the minor roadway can be handled with minimal interference to traffic approaching the intersection from any direction.
• Lower cost than other interchange configurations.
• Requires less right of way than other types of interchanges.

Disadvantages

• Possible wrong-way entry on the ramps from the crossroad must be addressed.
• The greatest impediment to smooth operation is the left-turning traffic at the crossroad intersections.
• Level of service decreases as the volume of traffic on the crossroad becomes heavy.

The simplest and perhaps the most common interchange configuration is the diamond. A full diamond interchange is formed when a one-way diagonal ramp is provided in each quadrant of the interchange. The ramps are aligned with free-flow from the main roadway and an intersection on the crossroad. Diamond interchanges have application in both rural and urban areas. They are particularly adaptable to major-minor crossings where left turns at grade on the minor roadway can be handled with minimal interference to traffic approaching the intersection from either direction. The intersection on the crossroad formed by the ramp terminal functions as a T-type intersection and is designed according to the direction outlined in At-Grade Intersections. However, because these intersections really do have four legs, two of which are one-way, they present a problem in traffic control to prevent wrong-way entry from the crossroad. For this reason, a median can be provided on the crossroad to facilitate proper channelization of traffic. This median can be either painted or raised. Additional signing is helpful to prevent improper use of the ramps.

Diamond interchanges may need additional traffic control when the crossroad carries moderate to large volumes of traffic. Traffic signals and roundabouts are options to consider.

• The capacity of the ramps and that of the crossroad may be determined by the signal-controlled ramp terminals. In this case, it may be necessary to widen the ramps, the crossroad or both to provide the necessary traffic capacity. While a single lane ramp may adequately serve traffic leaving the main roadway, it may have to be widened to two or even three lanes and combined with channelization for storage on the crossroad, to provide the needed traffic capacity at the ramp crossroad intersection. This design can prevent the line of stored vehicles on the ramp from extending to far along the ramp and onto the main roadway. Left-turning movements in most common diamond interchange configurations usually require multiphase signal control.
• Roundabouts at the ramp termini can offer advantages over signalization.

Diamond interchanges may be designed with or without frontage roads. The frontage roads are commonly used in developed areas. Frontage (or backage) roads are spaced a sufficient distance from the ramp to provide adequate weaving lengths, space for vehicle storage and turn lanes at the crossroad.

In a diamond interchange, the greatest impediment to smooth operation is the left-turning traffic at the crossroad terminal. Creative modifications (i.e. “splitting the ramps”, lengthening the ramps, moving the ramp terminal on the crossroad further from the main roadway, etc) to the basic diamond configuration may be necessary to improve the operation of the interchange.

Diverging Diamond Interchange (DDI) DDI Animation

A new design that illustrates this point is the Diverging Diamond Interchange (DDI).

The diverging diamond interchange allows for two-phase operation at all signalized intersections within the interchange. This is a significant improvement in safety, since no left turns must clear opposing traffic and all movements are discrete, with most controlled by traffic signals. Additionally, the design can improve the efficiency of an interchange, as the lost time for various phases in the cycle can be redistributed as green time; there are only two clearance intervals (the time for traffic signals to change from green to yellow to red) instead of the six or more found in other interchange designs. Some of the intersections in the design can be unsignalized. The left turn from the freeway off-ramp, for example, can form an auxiliary lane that then becomes an exit-only lane for the entrance ramp to the freeway in the opposite direction.

Advantages of a DDI

• Provides for two phase signals with short cycle lengths, significantly reducing delay.
• Reduced horizontal curvature reduces risk of off-road crashes.
• Increases the capacity of turning movements to and from the ramps.
• May potentially reduce the number of lanes on the crossroad, minimizing impacts to existing right-of-way.
• Substantially reduces the number of conflict points theoretically improving safety.
• Theoretically improves pedestrian safety.

Disadvantages of a DDI

• Drivers may not be familiar with configuration, particularly with regards to merging maneuvers along the left side of the roadway or the reversed flow of traffic.
• No standards currently exist for this design and the design is extremely dependent on site-specific conditions.
• Pedestrians would have to cross free-flowing traffic in freeway ramps. This could be mitigated by signalizing all movements while not impacting the two-phase nature of the interchange's signals.
• Additional signing, lighting, and pavement markings will be needed.
• There is no accident history available at this time since this is a new interchange concept.
• The local road should be a low speed facility (preferably posted speed less than 45 mph on the crossroad approach, but this may be mitigated by utilizing a higher design speed for the crossing movements).

Key elements in the selection of this type of interchange are:

• For higher traffic volumes, the DDI design has better performance and offers lower delays, fewer stops, lower stop time and shorter queue lengths when compared to the performance of the conventional diamond design. For lower volumes, performance of the DDI and a conventional diamond interchange are similar.
• A DDI has a higher capacity for all signalized movements when compared to the conventional diamond interchange. Especially, the capacity of left-turn movements is twice that of the corresponding capacity of left-turn movements of the conventional diamond interchange. Exclusive left-turn lanes are not necessary for the DDI.
• To be comparable to a 4-lane DDI, a conventional diamond interchange would require 6-lanes to provide the same capacity. When additional future capacity is needed, it would be advantageous to convert a conventional diamond interchange into a DDI instead of pursuing the more costly option of widening the major and minor roadways in the interchange (includes widening the bridge) and adding additional lanes to the ramps.
• While the DDI design does not allow through movements from off- to on-ramps, it permits u-turn movements with fewer conflicts than a conventional diamond interchange.
• The potential for wrong way traffic movements in a DDI can be minimized with geometrics, signing, pavement marking, signals and lighting.
• Nearby signalized intersections may reduce the effectiveness of a DDI. The simplified signal phasing of the DDI may impact the phasing of nearby traffic signals.

Access for all traffic movements at interchanges on the state highway system will be provided. In some cases, traffic volumes may not warrant the provision of all ramps at an interchange at the time of initial construction. However, when this occurs right of way will be purchased prior to initial construction that will provide for the “future ramps”. The plans for initial construction of the interchange will show the location of the “future ramps”, that will provide for these traffic movements.

234.2.1.1 Ramp and Preliminary Plan Criteria

Traffic capacities for ramp design are subject to variation and are limited by the geometric features of the ramp itself, the ramp termini, the weaving sections, the volume of through and turning traffic, and the intersection spacing within the functional area of the interchange. Access Management Guidelines are used for additional information. Procedures and methods for evaluating ramp capacities are given in the Highway Capacity Manual and A Policy on Geometric Design of Highways and Streets (AASHTO Green Book).

Ramp base lines are always equated to the survey centerline and other ramp base line intersection points or the crossroad centerline intersection point. The equations include offsets and intersection angles.

Interchange ramps are numbered for ease of identification. The width of each ramp should be as indicated on Standard Plan 203.40.

The district approves the geometric layout of a diamond interchange from the schematic drawing shown on the preliminary plan.

Drawings with detail interchange geometrics are developed to the same scale used on the plans and on standard size, 22 in. x 34 in. (560 mm x 865 mm) sheets. The drawing includes complete alignment details for ramp base lines and shows the limits of pavement. A supplemental standard size full profile sheet or cross section sheet is used for plotting profiles and developing ramp grades. The profile is plotted to the horizontal scale used on the plans, and to a vertical scale of 1 in.=10 ft. (1:100). If a standard size sheet is too small, additional standard size sheets are used, with match lines, or the layout may be prepared on a sheet of multiples of the standard size. The plan is developed to the best possible accuracy since this sheet becomes the base for later drawings (i.e. for grade separation reports). Grades are considered tentative at this stage and are developed in a similar manner to the preliminary plans. The plan sheet(s) includes complete traffic data for the interchange, including all turning movements. The traffic data is shown with the design hourly volume (DHV) over average annual daily traffic (AADT) and also the percent of trucks.

At this preliminary plan stage, two prints of the interchange geometric sheets are submitted to the Design Division for review and comment. Three prints are submitted for all interstate projects so approval can also be obtained from FHWA.

Alignment controls for diamond interchanges are available. The ramp curvature at the ramp nose or gore area will have a minimum radius of 6 degrees or less (295 meters). The delta for this curve will be as small as the ramp grade and location of the ramp terminal at the crossroad will permit. If practicable, the ramp curvature is established so both ends of the curve are at least 100 ft. (30 m) from the ramp nose measured along the ramp base line.

234.2.1.2 Basic Number of Lanes

A fundamental design control that is maintained along a main-line roadway in addition to the requirements of lane balance and capacity is the "basic number of lanes". The basic number of lanes is the constant or minimum number of lanes provided throughout a significant length of the main-line roadway. This number is exclusive of the number of auxiliary lanes. Indiscriminate adding or dropping of basic lanes is discouraged.

234.2.1.3 Coordination of Lane Balance

Proper lane balance is maintained on the main-line roadway at interchanges. The required number of lanes as determined by volume-capacity relations sometimes changes significantly at the entrance or exit points of the interchange. The following requirements for lane balance can generally be applied to both entrance and exit ramp traffic.

• At entrance ramps, the number of lanes beyond the merging of two traffic streams is not less than the sum of all traffic lanes on the two merging roadways, minus one.
• At exit ramps, the number of approach lanes on the main-line roadway must be equal to or greater than the number of lanes on the main-line roadway beyond the exit plus the number of lanes on the exit ramp, less one.

Chapter 10 of the AASHTO Green Book has additional information concerning the coordination of lane balance and the basic number of lanes.

Major fork and branch connection designs, with appropriate signing to designate the optional exit lane destinations, may be considered where appropriate as discussed in the AASHTO Green Book (sign examples are shown in Signing). Separate lanes with separate lane use signing are preferred where major forks occur.

234.2.1.4 Auxiliary Lanes for Acceleration and Deceleration (Parallel Type)

Minimum speed change lengths are given in Exhibits 10-70 and 10-73 of the AASHTO Green Book. Lengths shown in these tables are for grades of 2% or less on the speed change lane. Exhibit 10-71 of the AASHTO Green Book provides adjustment to these lengths for grades over 2%. Speed change lanes are provided at all ramp entrances and exits where the number of through traffic lanes each side of the ramp terminal are equal. The combined length of full width acceleration lane and taper will not be less than 600 ft. (180 m). A shoulder, at least 6 ft. (1.8 m) wide, is provided for auxiliary lanes along the through traffic lanes (except a 4-foot [1.2 m] shoulder will be provided along a median acceleration lane). Auxiliary lane width is the same as the width provided for the through traffic lanes. In rural areas, the ramp nose will be visible to approaching traffic for a distance equal to at least 1.25 times the stopping sight distance on the freeway of expressway. When Stoping sight distance cannot be obtained in rural areas a deceleration lane length of 350 ft. (110 m) is used.

Where interchanges are closely spaced (e.g. less than 2500 ft. [760 m]) the auxiliary lane for acceleration will be extended to the exit of the next interchange. An entrance lane followed by a lane exiting forms a traffic weaving section that requires added pavement width and length for weaving capacity. The capacity of the auxiliary lane connecting the on-ramp with the off-ramp will be determined using the Highway Capacity Manual or an appropriate traffic-modeling program (i.e. VISSIM). The weaving section will have a length and number of lanes based on the appropriate level of service outlined in the Facility Selection article.

Where a two-lane entrance ramp or a two-lane exit ramp is needed for capacity or lane balance, the effective length of auxiliary lane will be determined as illustrated in Exhibits 10-76 and 10-77 of the AASHTO Green Book.

234.2.1.5 Clear Zones

Clear zones will be provided on all ramps, where feasible. Specific guidance concerning clear zones is found in the Clear Zone article.

234.2.1.6 Lane Drop

Where a reduction in mainline traffic demand indicates a need for less traffic capacity, a lane drop or reduction in the number of through lanes is made on the exit ramp, preferably one with a high traffic volume. This reduction may be made provided the exit volume is sufficiently large to change the basic number of lanes beyond this point on the route as a whole. A lane drop is only made on a right hand exit ramp. Under no circumstance will a lane drop be made on a left hand exit ramp. This creates an unacceptable situation for safe traffic operation.

Where a lane drop or a reduction in the number of thruway lanes is made, an "escape lane" or a pavement taper of 50 to 1 convergence is provided beyond the gore nose for traffic to merge into the remaining through traffic lanes. Similarly, where two lane exit ramps are used, an "escape lane" is provided, if an auxiliary lane for lane balance is not provided beyond the gore nose.

234.2.1.7 Nose and Ramp Pavement Width Transition

Examples of ramp nose treatment and pavement width transition, including additional pavement widths for storage, are available.

234.2.1.8 Superelevation

Minimum controls for superelevation of short radius curves on ramps are shown below:

MINIMUM RADII AND SUPERELEVATION FOR TURNING SPEEDS
OPERATING SPEED MPH	RADIUS FT.	SUPER ELEVATION FT./FT.	LENGTH OF CIRCULAR ARC, DESIRABLE FT.
15	50	0.00	60
20	90	0.02	60
25	150	0.04	70
30	230	0.06	110
35	310	0.08	140
40	430	0.08	190
45	540	0.08	200

MINIMUM RADII AND SUPERELEVATION FOR TURNING SPEEDS
OPERATING SPEED km/h	RADIUS m	SUPER ELEVATION m/m	LENGTH OF CIRCULAR ARC, DESIRABLE m
15	7	0.00	20
20	10	0.00	20
30	25	0.02	20
40	50	0.04	20
50	80	0.06	40
60	115	0.08	50
70	160	0.08	60

A maximum super elevation rate of 6% is used when a short radius curve is on a bridge structure. This is the maximum superelevation suitable for satisfactory traffic safety under snow and ice conditions. The maximum rate of cross slope change is 5% per 100 ft. (30 m) to transition the superelevation cross slope back to normal cross slope. Ramp entrances and exits are designed to reach full superelevation at the ramp nose, if full superelevation can be obtained at this point. Superelevation transition for typical ramp entrances and exits are shown on standard plans. At ramp terminals with the thruway, the maximum algebraic difference in pavement cross slope is 5%.

234.2.1.9 Grades

The general grade layout of interchanges, such as crossroad over or under and ramp grades is selected and designed with grading economy in mind. The desirable maximum ramp gradient is 5 percent. In special cases, ramp grades as steep as 7 percent may be used. The use of grades steeper than 5 percent is usually restricted to short grades in urban or suburban areas. Vertical curves for ramps, both crest and sag, are designed to meet the requirements given in the Vertical Alignment article based on the desirable ramp operating speed, except that passing sight distance is not considered. The ramp grades for diamond-type interchanges at the ramp intersection with the crossroad are designed in accordance with these same requirements, regardless of whether the ramp grade is up or down in relation to the crossroad. Crossroad grades, ramp grades, and sight distances in the vicinity of diamond-type ramp intersections with the crossroad are developed in accordance with the requirements given in the At-Grade Intersections article. If the crossroad operating speed is unknown, or is less than 30 mph (50 km/h), an operating speed of 30 mph (50 km/h) is used. Mainline grades are set, if at all possible, prior to developing the geometrics and ramp grades. The best procedure is to develop grades through and adjacent to the interchange so the mainline grades outside the interchange area can be adjusted to finally balance grading quantities, and the crossroad through the interchange can be adjusted as necessary to provide near minimum vertical clearance for the grade separation based upon the final structure layout. This procedure will cause a minimum of revision to the geometrics as the grades are finally adjusted.

234.2.1.10 Grading

MicroStation, GEOPAK or other computer programs or aids are used to compute grading quantities in interchange areas. The method used is dependent upon the type of interchange, the terrain, and other site-specific factors. Regardless of the method, the plans will include adequate cross sections or contours to determine the quantity of grading material within the interchange area.

234.2.1.11 Site Grading

Grading quantities in interchange areas ( Class A, Class C or Unclassified Excavation) will include site grading necessary to properly handle drainage, to improve appearance, and to eliminate blind intersections. Where possible site grading is done for all interchange areas. Preliminary plans are reviewed carefully to determine if minimal site grading will address these items and yet provide economy in the design.

Where site grading of an interchange area is made, it is desirable to provide flatter slopes and smoother contour lines, particularly at diamond interchanges. At locations where the crossroad is over the mainline, ramp slopes on the mainline side will be carried from the mainline roadway ditch to the shoulder point of the ramp. This will cause a variable slope between the ramp gore and the crossroad. Slopes on the outside of ramps will not be steeper than 1:3 and carried back to its intersection with the mainline roadway.

With additional site grading and flatter slopes, guardrail would only be needed to protect bridge ends within the interchange area. The grading and slopes will also aid in reducing sign post size, length and location.

234.2.1.12 Drainage

The plans will provide adequate facilities for handling drainage through and from the interchange area, including adequate provisions to prevent water or melting snow from running across pavements. Erosion control in the interchange areas is also provided in the design. Storm water detention basins will be considered within an interchange in urban areas.

234.2.1.13 Typical Sections

The plans will include detailed typical sections for all ramps and the crossroad not covered on the plans for the mainline roadway. They will also include the location of survey and base lines, and the location of the profile grade in relation to the typical section.

234.2.1.14 Contract Plans

Contract plans show complete details for the construction of interchanges, including grading, geometrics, paving and drainage. Interchange grading quantities are tabulated on the plans, separated as much as conveniently possible from the main roadway grading quantities. Interchange grading quantities are considered in determining balance points in and adjacent to the interchange area. Details for typical ramp intersections are shown on standard plans. Similar details are required for ramp intersections not covered by the standard plans.

234.2.1.15 Signals

When warranted, the provision of traffic signals will be made early in the design process. Items to consider include lane assignments, signal visibility, pedestrian requirements and placement of the signal posts and bases. The preferred method of signal post design uses conventional single tubular steel posts, mast arm and bases as shown in the standard plans. This may be accomplished by using traffic islands, if necessary. All non-standard signal structures, shop drawings and stress calculations must be submitted to the Traffic Division for approval prior to fabrication. Detailed information is contained in 902 Signals.

234.2.1.16 Lighting, Signing and Pavement Marking

The placement and maintenance requirements of lighting, signs and pavement marking, must be considered early in the design process. Standard MoDOT equipment will be used.

Lighting:

• The most important design principles are uniformity of light and minimization of glare.
• The mainline and crossroads are well lighted
• Good lighting is provided at the ramp junctions.
• Detailed information is contained in 901, Lighting.

Signing:

Appropriate signing is provided in the interchange area to avoid motorist confusion. Important points to consider are:

• Overhead guide signing is recommended for the approaching roadways
• Traffic guide sign applications on the exit ramps are to be consistent with mainline signing.
• Use advance signing such as lane use signs over each lane on the roadway approaching the interchange. The sign support structure is placed at or just beyond the point where the turn lane is fully developed.
• Detailed information is contained in 903, Signing

Pavement marking:

• Appropriate pavement marking is provided in the interchange to avoid motorist confusion.
• Detailed information is contained in 620.2 Pavement and Curb Markings.

234.2.2 Maintenance and Emergency Crossovers

Maintenance and emergency crossovers may be constructed on freeways and expressways so that maintenance, emergency and law enforcement vehicles can avoid extremely adverse distances. These crossovers

• facilitate maintenance activities such as snow removal and
• provide ample access for law enforcement or emergency medical services responding to roadway incidents.

Maintenance crossovers may be needed at one or both ends of interchange facilities, depending on the type of interchange.

Information concerning their location and requirements for construction can be found at 240 Maintenance and Emergency Crossovers.