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Design of High Speed Arterial Median Acceleration Lanes

Description:

Median acceleration lanes are generally constructed with a parallel design with a taper length of approximately 92 m (300 ft.). Median acceleration lanes can be used both at T-intersections and at four-leg intersections. The use of a median acceleration lane at a four-leg intersection probably changes the turning paths and conflict patterns of opposing vehicles within the median roadway, but the extent of this effect is unknown.

Objective:

The objective of this research is to produce a set of proposed guidelines to aid practitioners in using median acceleration lanes.

Benefits:

The left-turn movement from a minor roadway onto a four-lane divided highway continues to be a safety concern and many studies have investigated ways to implement new strategies or eliminate confusing objects, lanes, or traffic control that may hinder the driver’s understanding of this complex situation. As stated previously, many divided highway intersections utilize a median acceleration lane to help drivers merge onto the mainline; however, the presence to aid drivers or its effectiveness to increase safety are not well understood.

This research study is expected to have significant payoff potential as many state highway agencies are in the process of improving divided highway intersections. Furthermore, if this study indicates effectiveness, research can further look into optimal design considerations or best practices to retrofit an acceleration lane into an intersection. Implementation of the research is straightforward in that recommendations found in the study can be immediately applied to practice.

Related Research:

The following is background information on median acceleration lanes from NCHRP Report 375.

Median acceleration lanes are increasingly used at intersections on high-speed divided highways. However, median acceleration lanes are not appropriate for all divided highway intersections. The 1994 AASHTO Green Book states the following on pages 750 and 751:

Acceleration lanes are not always desirable at stop controlled intersections where entering drivers can wait for an opportunity to merge without disrupting through traffic. Acceleration lanes are advantageous on roads without stop control and on all high volume roads even with stop control where openings between vehicles in the peak hour traffic streams are infrequent and short.

In wider medians, where the opposing left-turn lanes do not overlap, median acceleration lanes can typically be provided within the width used for the conventional left-turn lane in that same direction of travel. Thus, median acceleration lanes can be incorporated in many existing designs without widening the median. However, the presence of a median acceleration lane reduces the amount by which the conventional left-turn lane for the opposing direction of travel on the divided highway can be offset. Thus, providing an operational and safety advantage for left-turns onto the divided highway may create an operational and safety disadvantage for left-turns from the divided highways.

A 1986 survey by the Institute of Transportation Engineers (ITE) found that median acceleration lanes had been used by 13 of the 53 highway agencies (25 percent) that responded to the survey. Respondents to the survey were divided evenly for and against the use of median acceleration lanes. ITE concluded that the median acceleration lanes appear to promote efficient left-turns onto major roadways and to reduce accidents and traffic conflicts but that insufficient data are available to quantify their traffic operational and safety benefits.

On the basis of the guidelines used by state highway agencies, acceleration lanes for left-turning vehicles from a crossroad onto the divided highway should be considered at locations where adequate median width is available and the following are true:

  1. Limited gaps are available in the major-road traffic stream.

  2. Turning traffic must merge with high-speed through traffic.

  3. There is a significant history of rear-end or sideswipe accidents.

  4. ISD is inadequate.

  5. There are high volumes of trucks entering the divided highway.

One highway agency guideline stated that a truck volume of 75 to 100 trucks per day would be sufficient to warrant a median acceleration lane.

Median acceleration lanes, like offset left-turn lanes, have come into use at divided highway intersections in recent years but have been used by only a few agencies. A typical median acceleration lane is shown in Figure 9. The advantage of median acceleration lanes is that they allow vehicles turning left onto a divided highway to continue through the median roadway without stopping and merge onto the far roadway of the divided highway. That allows drivers to cross the near roadway of the divided highway without considering the availability of gaps on the far roadway, even if the median is not wide enough to store their vehicles; however, even if a median acceleration lane is provided, left-turning drivers must anticipate potential conflicts with other vehicles in the median area. Thus, the presence of a median acceleration lane changes the decision making process and the maneuvers made by the crossroad drivers turning left onto the divided highway, but not by drivers crossing the divided highway. The median acceleration lane should be long enough to allow left-turning vehicles to reach the speed of major-road traffic before merging.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:12 - 24 months
RNS Developer:Kay Fitzpatrick, Marcus Brewer, Eric Fitzsimmons
Source Info:Problem statement developed as a result of the Safety Effects of Geometric Design Decisions Workshop at the 2013 mid-year meeting of TRB Committees AFB10 (Geometric Design) and AHB65 (Operational Effects of Geometrics), in conjunction with the AASHTO Technical Committee on Geometric Design.
Date Posted:07/07/2014
Date Modified:07/15/2014
Index Terms:Arterial highways, Highway design, Median lanes, Acceleration lanes, Turning traffic, Four leg intersections,
Cosponsoring Committees: 
Subjects    
Highways
Design
Operations and Traffic Management
Safety and Human Factors

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