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Operational and Safety Impacts of Four- and Six-Lane Sections with Raised Medians Versus Two-Way Left Turn Lanes

Research Problem Statement


Multilane road cross sections are often designed to include some type of median, either depressed, raised, or flush.  When a flush median is selected, it often includes a two-way left turn lane (TWLTL).  In urban areas, the choice is often between a raised median and a TWLTL.  In some instances a designer would prefer a raised median in order to enhance mobility and safety, but abutting property and business owners express a strong preference for TWLTL.

Some have suggested various volume thresholds at what volume to convert from a five-lane design (TWLTL) to a non-traversable (e.g., raised) median.  Two concerns about non-traversable median designs are the additional travel distance and time due to the indirection caused by access restrictions, and the safety effects of the increased U-turn demand.


The analysis should consider and differentiate among the following factors:

·                     Number of through lanes: four or six

·                     Environment: rural, suburban, urban of various densities

·                     Volume and speed

·                     Signal density

·                     Access type and density


Literature Search Summary


Some of the existing studies are limited in scope, or otherwise do not address a full range of conditions and combinations of variables that need to be addressed.


·                    Safety Impacts of Selected Median and Access Design Features.  After determining that it was difficult to find suitable study sites, the researchers concluded that restrictive medians (flush grass or raised) were safer than non-restrictive medians.


·                    Investigation of The Impact of Medians on Roads Users, FHWA-RD-93-130.  This study examined the safety impact of raised curb medians, TWLTLs, and undivided cross sections on both vehicles and pedestrians in urban environments.


·                    Median Intersection Design, NCHRP Report 375.  This report developed guidelines for the selection of median widths for at-grade intersections.  It may provide insight into why there might be differences among different raised-median roadways.


·                    Access Management Manual.  This manual summarized findings from a number of studies about operational and safety impacts related to access management.


·                    Impacts of Access-Management Techniques, NCHRP Report 420.  This report documented the effects of various access management techniques, including median treatments.

·                    Safety of U-turns at Unsignalized Median Openings, NCHRP Project 17-21, draft final report under revision, as of October 2004.  This study examined the impact of U-turns on the safety of the road.


Research Objective


The objective of this research is to better document the trade-offs involved with selecting either a raised or a TWLTL median, and differentiate between these effects in a four-lane versus a six-lane environment.  The research should also incorporate the effects of different environments, volumes, speeds, signal densities, and access densities.


Estimate Of Problem Funding and Research Period


Recommended Funding: 



Research Period: 

30 months


Urgency, Payoff Potential, and Implementation


Design professionals need empirical data to assess and compare the safety attributes of non-traversable medians versus TWLTL’s for both four-lane and six-lane roadways at various volumes, speeds, and other characteristics.  The study will help determine under what conditions non-traversable medians should be required and help to sell non-traversable medians to the surrounding community when those conditions exist. With the emphasis on managing and improving traffic flow and safety, the need is urgent and the pay-off is substantial and immediate and applicable nationwide.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Date Posted:06/01/2007
Date Modified:06/04/2007
Index Terms:Raised medians, Medians, Highway safety, Traffic volume, Unsignalized intersections, Trip length, Two way left turn lanes, U turns, Highway operations, Left turn lanes,
Cosponsoring Committees: 
Operations and Traffic Management
Safety and Human Factors

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