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Highway Conflict

Conflicts are created along highways wherever driveways, intersections and other access points are introduced. These conflicts involve traffic traveling on the highway, crossing traffic, and traffic turning into or out of an intersecting roadway.

To reduce conflicts at these access points, right-turn and left-turn speed-change lanes are used. Speed-change lanes provide a separate facility for turning vehicles to decelerate or accelerate, and to queue while waiting to turn.

As a result, speed-change lanes minimize the interference between through traffic, crossing traffic, and turning traffic along the highway, creating a safe environment for the traveling public.

At rural unsignalized at-grade intersections, four types of speed-change lanes are generally used in Iowa:

  • Left-turn median deceleration lane
  • Right-turn deceleration lane
  • Left-turn median acceleration lanes
  • Right-turn acceleration lanes.

When are speed-change lanes warranted?

Warrants for the use of speed-change lanes cannot be stated definitely. Many factors must be considered, such as speeds, traffic volumes, percentage of trucks, highway geometry, frequency of intersections, and crash experience.

Deceleration lanes are advantageous on high speed roads because the driver of a vehicle leaving the highway must slow down in a through-traffic lane if a deceleration lane is not provided. The failure to brake by the following drivers because of a lack of alertness causes many rear-end collisions.

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