Acceleration and Deceleration Lanes

Introduction

Acceleration/deceleration lanes (also known as speed-change lanes or auxiliary lanes) allow drivers to speed up or slow down in a space not used by high-speed through traffic. On freeways and some major streets, the speed change can be large and cause stop-and-go traffic and collisions. Including acceleration/deceleration lanes in the roadway design can lessen these problems.

Dedicated acceleration lanes allow cars entering the main road to speed up to match the flow of traffic. Deceleration lanes allow exiting vehicles to slow down to a safer speed before making a turn at an intersection, without affecting the main flow of traffic.

Another configuration can connect the acceleration lane to the deceleration lane. After the freeway entrance ramp, the acceleration lane becomes an additional freeway lane. This lane then becomes an exit-only lane at the next exit (a deceleration lane). Acceleration/deceleration lanes reduce the effects of exiting and entering traffic on the main lanes.

The proper use of acceleration/deceleration lanes:

• Increases the average speed on freeways and major streets.
• Reduces the delays on ramps.
• Increases safety by reducing the number of conflicts between slow-speed and high-speed vehicles.

Executive Summary

 

Freeway Interchange Ramps
Acceleration/deceleration lanes on freeway interchange ramps can reduce congestion by creating specific areas for merging traffic to speed up and exiting traffic to slow down. These lanes reduce the stop-and-go effects and collisions caused by slower traffic on interchanges.

Areas between Freeway Entrance and Exit Ramps
Acceleration/deceleration lanes used between consecutive entrance and exit ramps allow traffic to speed up and slow down without affecting freeway traffic. These lanes work best on entrance and exit ramps that are short, requiring entering traffic to merge quickly and exiting traffic to slow before leaving the main traffic lane.

Major Streets with High Speeds and Turn Volumes
Major streets and frontage roads without acceleration/deceleration lanes or turning bays have congestion from traffic slowing down to turn or speeding up after turning. The differences in speed of through traffic and entering/exiting traffic can greatly slow traffic, cause stop-and-go traffic, and increase collisions.

• Encourages smooth increases in traffic flow, speed, and volume by allowing vehicles to reach the proper speed in a designated area rather than in the main traffic lanes. For example, exiting traffic moves into the specified area to slow down.

• Increases intersection capacity and efficiency. Acceleration/deceleration lanes allow turning traffic on major streets to move out of or into the roadway without interrupting the flow or wasting signal time.

• Improves the safety of ramp areas by allowing vehicles to reach the proper speed before merging into traffic. The reduced interference decreases the possibility of congestion and collisions.

Issues

Lane space and right of way are the main design issues with adding acceleration/deceleration lanes. Changing the current shoulders to useable lanes may require making the pavement stronger and wider. If the shoulder cannot be used, the road will need to be widened. This may require more right of way and result in higher construction costs. Complex, dated, or elevated designs make the project even more difficult and costly. Right-of-way limits at intersections may require a complete rebuild or different design.

The greatest project barrier may be assigning institutional responsibility. Few departments of transportation have staff assigned to look for locations where low-cost treatments can be installed. The contributions that acceleration/deceleration lanes might make are overlooked in favor of larger or more sophisticated programs.

Project Time Frame

The timeline for adding acceleration/deceleration lanes differs based on:

• The method used.
• The current road geometry.
• The roadway class.

A typical major street can be changed in a short time frame (perhaps one to two months) than an elevated freeway section (as much as a year). Cost, complexity, design, and benefit should be considered when adding acceleration/deceleration lanes to a roadway.

Who Is Responsible?

The local TxDOT office and cities bear the primary responsibility of installing and maintaining acceleration/deceleration lanes. TxDOT may take responsibility on state roads. On city roads, however, the local government controls the construction and management of turn lanes and acceleration/deceleration lanes. Improvements should be coordinated with local citizens and businesses in either case to ensure that the road serves the nearby land.

Cost

The cost of adding acceleration/deceleration lanes is based on many factors. Costs can range from $50,000 to $100,000 for a simple shoulder change to $1 million or more for more complex changes. Changing shoulders to acceleration/deceleration lanes costs less than widening the roadway, which may require increased right of way. Also, any construction on an at-grade street costs less than on an elevated freeway due to the design, construction time, and material costs.

Data Needs

Useful data for adding acceleration/deceleration lanes include speed changes in the main traffic lanes caused by slowing, exiting vehicles. Traffic counts on freeway exit and entrance ramps can help determine where large entering and exiting volumes occur. Turn counts on major street intersections can help determine the number of vehicles slowing down or speed up. Measuring travel delays before and after the start of this technique can help evaluate its effectiveness. Also, the current lengths, widths, and presence of the lanes and the entrance and off ramps will be needed.