Semisequicentennial Transportation Conference Proceedings
May 1996, Iowa State University, Ames, Iowa

The Super Two

Dennis R. Eyler and Alex Poletz

D.R. Eyler, Strgar-Roscoe-Fausch, Inc.,
One Carlson Parkway, Suite 150,
Minneapolis, Minnesota 55447.

A. Poletz,
Minnesota Department of Transportation,
Office of Technical Support,
395 John Ireland Boulevard,
St. Paul, Minnesota 55155.

In recent years there has been a revived interest in the operation of two- lane rural and suburban roadways. This renewed interest has developed due to increased traffic volumes and reduced funding levels. There is a growing need to maximize the capacity, mobility, and safety of existing two-lane roadways. New roadway corridors are also constrained by funding and space limitations as well as environmental concerns. The era of constructing new four-lane roadways with wide rural design medians to accommodate far off future traffic volumes is drawing to a close. A product of these reduced expectations is the concept of the "Super Two." The term Super Two can refer to either a freeway type roadway or a controlled access surface roadway (expressway). The Super Two concept is not a totally new, however. Two-lane freeways have been constructed in many locations over the years. Wisconsin State Route 29 east of Chippewa Falls has segments of two-lane freeway. There have even been sections of the Interstate system that have been initially opened to traffic as two-lane facilities. I-29 in North Dakota was an example. Key features of the Super Two concept are full width lanes, full width shoulders, frequent passing lane locations and the extensive use of right turn lanes, left turn lanes, and continuous two-way left turn lanes where warranted. The Minnesota Department of Transportation (Mn/DOT) has built several roadways that meet many of the Super Two requirements for at-grade expressways. Examples are T.H. 169 in Brooklyn Park, T.H. 101 between Rogers and Elk River, and T.H. 60 west of St. James. This paper outlines an organized set of design guidelines for creating both freeway and expressway Super Twos, discusses operations issues, and proposes new roadway width requirements and new pavement marking treatments to enhance the safety and capacity of the Super Two roadway.


There is an increasing interest in designing two-lane roadways with improved traffic operations features, in response to increasing traffic volumes and reduced funding. Capacity, mobility, and safety for existing two-lane roadways also need to be improved. However, the development of new corridors will continue to be constrained by funding limits and environmental concerns. The era of constructing new roadways with wide medians to handle possible future traffic volumes has ended. A response to these current realities is the concept of the Super Two.

DEFINITION

Super Two refers to a freeway or controlled access at-grade roadway with a single through lane per direction. The design features of the Super Two work together to maximize the capacity of that single lane. The key defining elements of a Super Two include:

  1. Full width lanes, paved shoulders, and clear zones
  2. A center buffer area
  3. Limited access, with turn lanes for all permitted turns
  4. Horizontal and vertical curves with high design speeds
  5. Passing lanes, speed differential, and truck lanes
  6. Provisions for expansion to freeway or divided roadway
  7. Proper interchange design for a two-lane freeway

For a new facility to be classed Super Two, most guidelines should be met. For upgrading an existing roadway, these defining features can serve as a menu of improvements for consideration.

The Super Two should be thought of as a distinct class of roadway. The term should create a specific image when used by the transportation profession and it could come to be a term also used by the driving public.

HISTORY

The Super Two concept is not totally new. There are many roadways that meet many Super Two requirements. Two-lane freeways have been built, either as a first stage or as a final product, and some two-lane at-grade roadways have the features of the Super Two. These versions of the Super Two have usually been considered only as interim steps to full four-lane freeways. However, when a two-lane has been designed as a temporary, often problems were created. The Super Two is intended to work smoothly and safely for a long time as a two-lane facility and yet be easily expandable to a larger roadway.

DESIGN PHILOSOPHY

The Super Two philosophy begins with it being a distinct classification of roadway falling between two-lane roadway and four-lane expressway or freeway. If built as a two-lane freeway, it would have the capacity of a four-lane at-grade divided roadway. If built at-grade, it would have the capacity of a four-lane undivided roadway. In planning a regional road system, the Super Two would be a type of facility that would typically be used for minor arterials and volume principal arterials.

DESIGN FEATURES

Lane Width, Shoulders, and Clear Zones

Lane widths for a Super Two are 12 feet (3.6m). Shoulders are paved and 10 feet (3m) wide. Clear zones are provided, ideally for the ultimate roadway section, where cost considerations allow.

Center Buffer Area

The most notable feature of the Super Two, is the center reaction (buffer) area. This buffer consists of a 6.5-foot (2m) area between opposing flows where traffic is allowed to cross only during passing. This feature is key to the concept. Just providing wider lanes would not accomplish the goals of improved traffic flow and safety. In Europe, efforts toward a Super Two design used wider lanes and did not produce lower accident rates.

The buffer area is a logical advance in creating safer highways. There is a continuing effort to eliminate fixed objects from the right side of roadways to provide clear zones and safety slopes. However, on the left on a two-way roadway, there are oncoming vehicles within a few feet. Roadways have been designed with consequences of straying left that are much more severe than those for straying right. On two-lane roads there is also air turbulence caused by large trucks. Sight distance to oncoming traffic for passing would be better with separation between opposing traffic lanes and center line crossing on tight curves would be reduced. The buffer area also assists the transition from an undivided roadway to one having raised medians.

Access with Turn Lanes

At-grade Super Two access is limited to intersections spaced at one-half mile (800 m) or greater. Super Two freeway design has spacing of one mile (1600 m) or greater between interchanges. Side street approaches, have controlled access for 500 feet (150 m) to limit driveways near intersections. Access is not allowed without turn lanes, particularly left turns.

Left turns made from a through lane create a large speed differential between through traffic and vehicles slowing or stopped to wait for gaps. There have been attempts to use bypass lanes in lieu of providing left and right turn lanes. Bypass lanes cause problems if left and right turning vehicles are both present. Bypass lanes also do not provide an adequate view of oncoming through traffic when there is an opposing left turning vehicle.

Design Speed For Alignment

The Super Two can evolve into a freeway. Therefore alignment should meet freeway standards. In addition, vertical curves should provide passing sight distance wherever practical.

Passing, Speed Differential, and Truck Climbing Lanes

Traffic volumes and sight distance limitations will determine the number and length of passing lanes. Passing lanes increase capacity, reduce accidents and are a motorist service. The need for truck climbing lanes should be thoroughly investigated for they become more crucial for a single-lane operation. A slow truck can equal up to 10 cars. Another related feature is the speed differential lane. This is an added lane that enables slow vehicles to be passed while traffic is reaching highway speeds after leaving a "STOP" or signalized intersection, a railroad crossing, or a town.

Design Provisions For Expansion

In the Super Two design, the ultimate section must be considered. Care must be taken in laying out intersections, particularly for concrete pavement. Superelevation, water run-off, and drainage structures become issues if future expansion is to be made equally about the centerline.

Proper Interchange Design

If the Super Two is a two-lane freeway, interchanges must be designed carefully. A ramp entering a single lane cannot operate as it does when entering a multi-lane roadway. On a multi-lane roadway, traffic can change lanes to avoid entering vehicles. With a single lane, changing lanes would spell disaster. Interchanges on two-lane freeways must have medians, multiple lanes, or parallel acceleration lanes for the entering ramps.

ISSUES

The key issues for the Super Two include costs, the center buffer, traffic control, no access without turn lanes, and the likely occurrence of high speeds.

Costs will only be slightly more than for a typical modern two- lane roadway. For any project of this type a major investment study would be the proper means to determine if Super Two is a cost effective transition to the ultimate design.

Center buffers such as two-way left turn lanes and painted medians on four-lane roadways have been used and have shown to reduce head-on and sideswipe accidents.

The principal issue of traffic control will be marking and signing the buffer area. An appropriate but unique striping system will need to be developed. For signing, informational signs and two-way traffic warning signs will certainly be needed. Traffic signals must be traffic actuated with high speed detection layouts and recommended to have active advance warning flasher systems.

Allowing access only with turn lanes may seem extravagant when thinking of field entrances. However, field entrances may grow to something else. It is the infrequent left turns that are the greatest risk for rear-end collisions. In addition, settling access locations at the earliest time is best.

The possibility of higher speeds is real, but traffic in single file is restrained somewhat by slower vehicles. This design is intended for high type roadways and speeds of 60 miles per hour (100 km per hour) would be acceptable.

CONCLUSION

This paper has been prepared primarily to generate discussion of this subject. It is not intended to establish design standards. Often the best final products come from reacting to what was initially perceived as an off-the-wall suggestion.

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