Institute for Transportation

Project report, 14 short summaries respond to commonly asked questions and misconceptions

There’s such a large quantity of guidance literature and research documents available about four- to three-lane roadway conversions that a major challenge for practitioners is easily accessing the most relevant information.

A recently finalized project led by the Iowa Local Technical Assistance Program (LTAP) aims to help those practitioners by taking the available information and using it to succinctly and directly answer some of the more commonly asked questions related to the planning, design, and/or application of four- to three-lane conversions, also called road diets.

In total, the project sponsored by the Iowa Department of Transportation (DOT) and Iowa Highway Research Board (IHRB) and in consultation with the state’s practitioners provided responses to 14 commonly asked questions related to the conversion of four-lane undivided roadway to three lanes (i.e., one lane in each direction and a two-way left-turn lane).

“The information in a number of these responses may also be relevant to decision-making related to other roadway cross section conversions or roadway improvements,” said Keith Knapp, who is the Iowa LTAP director and principal investigator on the project.

The responses are encompassed in a final report, which also addressed five common myths or misperceptions and included recommendations for future work, and they are provided as standalone documents for practitioners to select the response to their specific questions.

“The subject material of these questions is far ranging and includes, among other things, planning-level factors, safety impacts, and considerations related to operations, railroads, and winter maintenance,” said Knapp.

The following are just a few of the 14 questions answered in the two- to five-page summaries:

• What can average daily traffic tell me about the potential outcomes of a four- to three-lane conversion?
• What are the potential safety impacts of implementing turn lanes and two-way left-turn lanes?
• What are some parallel facility and treatment options that can serve pedestrians and/or bicyclists along a roadway?

While a wide variety of questions are answered, the report also notes that additional subjects and other issues are likely to arise and evolve over time and recommends that additional questions and summary responses be created as interest dictates.

Other recommendations include creating short summaries on four- to three-lane conversions with elected officials as the target audience and updating the summaries with any new and Iowa-specific information.

Iowa and Knapp have been leaders in publishing research related to four- to three-lane conversions, starting with state guidelines developed in 2001. National guidance followed in 2014 thanks to research led by Knapp and sponsored by the Federal Highway Administration (FHWA), which considers the application of four- to three-lane cross section conversions a proven safety countermeasure.

To read the final report and the 14 standalone summary responses, visit the research project page.