Not enough credit is given to 4 to 3 lane road diets. They can be a relatively low cost way to calm traffic, reduce crashes and provide bicycle lanes all in one resurfacing project.
4 lane roads attempt to accommodate higher traffic volumes and left turning movements by making inner traffic lanes de facto left turn lanes. This creates multiple traffic conflicts as through traffic and turning vehicles have to jockey within this inner lane. 4 lane roads also create multiple threat crash risks for pedestrians and generally higher traffic speeds. A 2014 article explained the issues with 4 lane roads in detail and created a lot of discussion about conversions in Minnesota. Many of the same things can be said in cities across America: Better utilization of right of way on 4 lane cross sections can create safer streets which are more accessible to all road users.
Re-striping a roadway with 2 bi-directional traffic lanes and a middle turn lane creates a dedicated, predictable space for left turns while freeing up right of way for bike lanes, wider sidewalks, pedestrian refuge islands, or on-street parking. FHWA has put out a comprehensive guide that explains many benefits of these conversions.
Best of all, many of these benefits can be created with re-striping only. Keeping curbs in place is a great cost saver and makes 4 to 3 lane road diets more practical for a greater number of cities.
Many conversions have already been studied. Significant crash reductions have been recorded with notable reductions in excessive (above speed limit) traffic speeds. Surveys show livability and pedestrian comfort benefits and some economic development/vacancy rate benefits, too. Project locations should be carefully considered though as road diets aren’t a panacea. Traffic volumes, crash types, adjacent land uses, and the location of the project within the larger road network all need to be considered during the planning phase.
The traffic threshold for 4 to 3 lane conversions varies by city but is usually around 20,000 vehicles a day. The good news is that many existing 4 lane roads are significantly under capacity and were designed based on (over-estimated) 2%-3% traffic growth rates per year which never materialized. Assuming the traffic threshold is met, conversions can provide benefits to rural, suburban and urban mixed use streets at a fraction of the cost of full reconstruction projects.