Traffic and Safety

4– to 3–lane conversion


What is a  4- to 3-lane conversion?

A 4- to 3-lane conversion (a type of road diet) involves restriping a 4-lane road as a 3-lane road. One lane is provided in each direction with a continuous left turn lane down the center. This often leaves room left over which can be used for enhancements such as parking or bike lanes. When combined with a roadway resurfacing project, a 4- to 3-lane conversion can be completed without any additional cost. 4- to 3-lane conversions are one of the Federal Highway Administration’s (FHWA’s) 20 proven safety countermeasures.

4 to 3 lane example
4 to 3 lane example

What are the benefits of a 4- to 3-lane conversion?

  • Shown to reduce crashes by about half.
  • Decreases left-turn and rear-end crashes because of the dedicated center turn lane.
  • Opportunity to provide enhancements such as parking or bike lanes.
  • Improves walkability - fewer lanes for pedestrians to cross, and traffic is located farther from the sidewalk.
  • Easier access to homes, schools, and businesses.
  • Regulates speeds and calms traffic. Speeds are more consistent with just one lane in each direction.

When should a 4- to 3-lane conversion be considered?

Map  Cities with at least one 3-lane road

There are many factors that should be considered before implementing a 4- to 3-lane conversion. Some of the factors that the Iowa DOT considers are traffic volume, number of intersections, type and location of access points, as well as a corridor’s crash history.

Many communities in Iowa have had success implementing 3-lane roads in urban, suburban, and rural locations.

To proactively identify where future conversions might be considered, the Iowa DOT recently conducted a
statewide screening. This effort provided a snapshot of potential locations where a conversion might be successful.