Loop Ramps Reducing Left Turns

Introduction

Bottlenecks can occur when a lot of vehicles exiting a freeway want to turn left at the upcoming intersection. Setting the timing of the signal at that intersection to relieve the left-turn congestion may worsen traffic for the crossing street. One solution to reduce delay is to put the left-turning traffic on a loop ramp to access the cross street instead of turning left at a traditional intersection.

The loop ramp design is a practical solution if sufficient right of way is available. The loop design changes a potential stop at the intersection into a merge or yield movement. Collector-distributor roads can be used with loop ramps to provide a speed-change area for vehicles traveling between the cross streets.

Executive Summary

Freeway Entrance/Exit Ramp Intersections
Making space for a lot of left-turning vehicles can greatly affect the operations at a signalized intersection. These movements can be removed if a loop ramp is provided. On freeways without frontage roads, providing a loop ramp allows the left-turn phase at the signal to be eliminated.

For freeways with frontage roads, left-turn phases must still be provided at the signal for vehicles traveling to or from locations along the frontage road. However, the left-turn phase time may be greatly reduced.

Major Intersections with Heavy Left-Turn Volumes
Loop ramps can improve traffic at major intersections that have heavy left-turn volumes. In this case, the loop ramps would be installed as part of a grade-separation project. This type of application is sometimes used as part of a highway-rail grade-separation project. In this case, additional benefits are realized through the removal of the highway-rail grade crossing.

• Reduces delay, improves capacity through shorter cycle lengths, and simplifies signal timing. At an isolated signalized intersection, eliminating a protected left-turn phase increases capacity by reducing lost time. It may even allow the signal cycle length to be decreased. At a signalized diamond interchange, if left-turning movements are removed because of a loop ramp, it becomes easier to implement the three-phase or two-phase diamond interchange timing sequences. They better allow for improved traffic flow through the interchange and to adjacent signals on the main street. Capacity is also increased by removing spillback that could occur due to waiting left-turn vehicles.

• Improves the efficiency of turning movements. When a free-flow loop ramp is provided, left-turning drivers no longer have to stop at a signal or stop sign. They may not even have to stop when merging onto the cross street if they are given a dedicated receiving lane. When a non-free-flow loop is provided, the movement is accomplished with a right turn instead of a left turn. Efficiency is also improved by eliminating the need to store left-turning vehicles waiting between the two ramps.

• Increases safety by reducing the number of conflict points. An intersection of a one-way frontage road and a two-way major street has 13 conflict points. Providing a loop ramp for one of the left-turn movements eliminates three conflict points. Providing a loop ramp for two of the left-turn movements eliminates five conflict points. Safety is also improved by eliminating left-turn vehicle queuing.

Loop ramps may be installed to accommodate one or both of the left turns at a signalized freeway ramp intersection. These two options are shown On the western side of the interchange, a loop is provided for the west-to-south movement. The south-to-east movement is still routed through a signalized intersection, but the signal does not need a protected left-turn phase for vehicles entering the freeway. A signalized intersection on the eastern side of the interchange is eliminated because loop ramps are provided for the north-to-west and east-to-north movements.

The decision to install loop ramps is based on operational concerns like left-turn volumes and queue length. Geometric concerns, like the availability of right of way, also play a part. In the left portion of the photo below, right of way was available in only three of the four sections of the interchange, so three loop ramps were built.

When two loop ramps are provided on one side of the freeway, a weaving area is created where exiting and entering vehicles cross paths. Including a collector-distributor road allows the weaving activity to be removed from the freeway main lanes. This option is shown in the left portion of the photo. Another design option is to braid the loop ramps, as shown in the right to eliminate the weaving movement.

For freeways with frontage roads, loop ramps can be added for left-turn movements entering or exiting the freeway. However, regular intersections are still needed for left-turning vehicles traveling to or from locations along the frontage road. In this case, it is not possible to eliminate the signalized intersection unless the frontage road volumes are very light.

The additional right of way and paved surfaces required to build collector-distributor roads and loop ramps greatly affect cost. The right-of-way requirements are influenced by the size of the loop ramps, which is based on design speed. A Policy on Geometric Design of Highways and Streets⁵ suggests a curve size of 150 to 540 feet for design speeds of 25 to 40 mph. It also states that design speeds on the higher end of this range should be chosen for freeway exit ramps, meaning a larger loop ramp and more right of way. Therefore, the cost of right of way increases rapidly with increasing design speed.

Adequate signing for road users is important to the success of a loop ramp. Proper signing is especially important for freeway entrance loop ramps because these ramps require drivers to access the ramp from the right side of the main street. Since these drivers are making a left-turn movement, their expectation is to access the entrance ramp from the left side of the main street. Signs for freeway exit loop ramps must specify both the name of the main street and the direction of travel served by the ramp.

The use of collector-distributor roads can make exiting and entering decisions more difficult. A lack of signing and marking can negatively affect drivers’ decisions. The anticipated speeds and the available sight distance on the approach to and within the weaving section should be considered during design.

Delay benefits from the use of a loop ramp are partially offset by the increased travel time on the loop ramp. The increased travel time is calculated as geometric delay in the Highway Capacity Manual.¹

Who Is Responsible?

State department of transportation officials are responsible for ramp construction projects on state-maintained highways. City officials are responsible for projects on city streets. In the case of an intersection between a state highway and a city street, coordination between state and city officials is needed at all stages of the project—planning, design, and construction. Shared operational management is needed if the signalized intersections at the interchange will coordinate with the nearby signals along the main street.

Project Time Frame

The time frame for the addition of a loop ramp depends on the scope of the project and the need for right-of-way purchase. The listed implementation examples suggest that adding loop ramps, along with straightening and widening other ramps at the interchange, can be completed in one to two years. The time frame will be longer if a collector-distributor road or braided ramps are included in the project.

Cost

Project cost may vary greatly depending on:

• The need to gain right of way.
• The number of loop ramps to be added.
• The desire to include a collector-distributor road or braided ramps.

The implementation examples suggest that adding a single loop ramp costs $2 to $4 million. If right of way must be purchased, costs can increase to $15 to $25 million.