Narrowing
Center Islands

Center islands usually narrow the travel lane at the roadway centerline and separate opposing traffic movements.

Center islands are usually raised islands, also called medians, within the roadway centerline. Center islands usually narrow the travel lane at the roadway centerline and separate opposing traffic movements.

When landscaped, center islands can improve the aesthetics of the corridor and can act as a gateway when placed at endpoints of a special area

Advantages

  • Provides physical separation between travel lanes
  • Creates refuge areas for pedestrians
  • May reduce head-on collisions

Disadvantages

  • Depending on the roadway cross-section, separated lanes could lead to higher speeds because drivers have less friction with on-coming vehicles
  • Reduced access to businesses
  • Requires additional long-term maintenance needs
  • May impact drainage

Effectiveness

Median treatments may be more effective when they provide a short interruption of traffic flow than when long center islands are used.

The Crash Modification Factors Clearinghouse provides crash modification factors (CMFs) of 0.56 for fatal and major injury crashes and 0.29 to 0.61 for all crashes after installation of a raised median.

Appropriateness

Rural main streets must have adequate additional right-of-way for center islands. If they do, center islands are especially appropriate when turning movements should be restricted or channelized downstream of an intersection or in areas that would benefit from a pedestrian refuge.

Design consideration for areas with heavy truck volumes or prevalence of large farm vehicles should be made so that larger vehicles can utilize the roadway.

Cost

The use of a raised median is a significant capital improvement. Use of pavement markings may be a fairly inexpensive alternative to raised medians.

Image: Center island acts as a gateway, separating the flow of traffic in town

Center island in Drakesville, Iowa (Shauna Hallmark et al./Institute for Transportation)

Image: Pavement markings create medians separating the flow of traffic in each direction entering and leaving town

Use of pavement markings in Union, Iowa, as an alternative to raised medians (Shauna Hallmark et al./Institute for Transportation)


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