PacTrans: Performance of Depressed Medians on Divided Highways in Alaska

AIDC project number:


Guhlam Bham



  • Start Date: Jul 1, 2013
  • End Date: Jul 31, 2015

Project Documents

Project Final Report

Project Summary

The population of Alaska especially the city of Anchorage is steadily increasing. As a result, traffic volumes are higher and demands to add lanes to existing highways is increasing in order to relieve congestion. In Alaska, an expressway or freeway is a high-speed (≥ 50 mph), multilane, divided highway with partial access control. These divided highways typically utilize wide medians. These medians are considered an option to be replace with narrow medians in order to provide additional through lanes to cater to the needs of higher traffic volume and keep traffic congestion to a minimum. During the Alaska winter where snow accumulates in medians from October to April, drivers can lose control of their vehicles in slippery road conditions; however, these wide medians with heavy snow serve as a refuge to absorb the impact of a crash. The damage to a vehicle and its occupants can be minimal as a result. If the existing wide medians are used to construct additional lanes, these wide medians will no longer be available to act as safety cushions. On the contrary, a narrow median will mainly act as a barrier to separate vehicles traveling in the opposite direction. Without a depressed median (a vee ditch), single vehicle run-off-road crashes could evolve into multi-vehicle crashes among vehicles traveling in the same direction. These depressed wide medians also act as snow storage areas, allowing snow to be plowed on both sides of the road. Eliminating these medians will reduce the available snow storage space and require plowing all of the snow to the right and increase the snow load to one side of the road. A study is proposed that will examine the operational and safety benefits of 1) keep existing wide medians, and 2) replace existing medians with a) concrete barriers, b) high tensioned cable barriers, and c) another feasible alternative. Modern protective devices, such as cable barriers, offer protection with a lower risk of vehicle damage and personal injury than do traditional concrete barriers and guardrails. The study will evaluate the benefits of travel way that is safer to use. Further, the study will examine the benefits of higher traffic volume with minimal congestion to the cost of construction of additional lanes and property damage to vehicles as a result of collision with different types of barriers.