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Whether you drive, bike, walk, or use public transit—safety on our nation’s roadways is paramount. In the 1950s and 1960s, roadway projects focused on system and capacity expansion. When the traffic volume of a two-lane road exceeded what it could accommodate, more lanes were added. This was a typical solution to address congestion issues, and as a result, four-lane roads became the norm across the country. But it turns out, more lanes are not always the best solution.

“Four-lane, undivided roadways often operate similar to two-lane roadways with left-turn lanes. People often shy away from using inside lanes because of the waiting potential, caused by left-turning drivers,” explains Tony Boes, PE, PTOE, Principal Traffic Engineer for Snyder & Associates. “Inside lanes become de facto left-turn lanes.”


Road Diets Defined

A Road Diet refers to the removal of vehicle travel lanes while maintaining the same roadway width. The most common form of a Road Diet, also known as a lane reduction or reconfiguration, converts a four-lane, undivided road into a two-lane road with a center, two-way left-turn lane.

A graphic showing the results of a 4-to-3 lane road diet.


“Road Diets change how roadway space is allocated. It uses the same amount of pavement, but moves the stripes around,” adds Snyder & Associates’ Senior Traffic Engineer Brian Willham, PE, PTOE, ENV SP. “By giving left-turning traffic its own space and through traffic its own space, you only need three lanes, not four. The extra space created by a Road Diet can be used for parking spots, bike lanes, and other complete street improvements that encourage active transportation and support the local economy. ”

Benefits of Road Diets

The Federal Highway Administration (FHWA) reports that four-lane, undivided roads have a history of high crash rates. For roads where collisions and speeding are common, or in sensitive areas near schools, parks, and neighborhoods, Road Diets provide significant benefits including:

  • An overall crash reduction from 19 to 47 percent.
  • Reduction in the likelihood of rear-end and left-turn crashes with a dedicated left-turn lane.
  • Fewer lanes to cross for side-street motorists, reducing the rate of right-angle crashes.
  • Pedestrian safety increases with fewer lanes to cross. Dedicated bicyclist/pedestrian space also helps increase driver awareness of other roadway users.
  • Through the reallocation of space, designated bike lanes, on-street parking, pedestrian refuge islands, and transit stops are possible.
  • Calming traffic and reducing speed helps reduce crash risk and the severity of collisions.
  • Turning gap selection for left-turns is simplified. This is especially important for inexperienced and older drivers as these age groups experience higher rates of traffic crashes.
  • If a driver departs the road, wider shoulders offer recovery space. This area also allows buses and mail or delivery trucks to depart from traffic, so vehicles can pass.
  • Increased economic vitality may result by creating a destination for all users. When on-street parking, walking areas, and bicycles lanes are available, people are more likely to explore and enjoy the area.

“The number one benefit of a Road Diet is safety,” says Willham. “By reducing the number of lanes, you don’t have through traffic mixing with turning traffic as much. As a result, we often see a significant reduction in rear-end, side-swipe, and left-turn crashes.”

Road Diet Feasibility Determination

Even though Road Diets increase safety and help foster a cohesive transportation network, they aren’t appropriate or feasible in all instances.

“A Road Diet isn’t an automatic, one-size-fits-all solution,” states Mark Perington, PE, PTOE, Traffic Group Leader for Snyder & Associates. “From the number of driveways and intersections in a corridor to right-of-way availability and cost, there are many factors to consider before moving forward with a Road Diet.”


When evaluating a corridor to determine if a Road Diet is appropriate, an understanding of your community’s goals for improvement is helpful. Common objectives include:

  • Improving safety
  • Reducing speed
  • Mitigating queues associated with left-turning traffic
  • Improving the pedestrian environment
  • Improving bicyclist accessibility
  • Enhancing transit stops

“To determine Road Diet feasibility, we begin by looking at the existing traffic volume and crash history of the corridor,” explains Boes. “We may also recommend a detailed analysis of existing conditions to see what’s working and what’s not while taking into account the growth potential of the area.”

An Economical Solution

As funding for roadway projects becomes increasingly limited, the desire for cost-effective, smart solutions is stronger than ever. The primary expense associated with Road Diets is road signing, markings, or signal modifications, so they’re fairly inexpensive and can be planned in conjunction with other maintenance efforts like resurfacing. In addition, Road Diets occur within the existing roadway footprint. This may reduce or eliminate costs associated with large reconstruction projects.

Federal funding for Road Diets includes the Surface Transportation Program or Block Grant (STP/STBG), the Highway Safety Improvement Program (HSIP), and DOT state-level traffic safety programs, among others where crash data supports the expenditure.

“Unless you have to improve existing pavement, there’s typically not any pavement construction cost, which can save a significant amount of money on transportation projects,” says Boes.

Education & Public Outreach Guide Success

According to the FHWA’s Road Diet Informational Guide, Road Diets have become more common since the 1990s with some of the first installations occurring in Iowa, Minnesota, Montana, as well as other states. The Snyder & Associates traffic and transportation teams were at the forefront of Road Diet installation in Iowa and have continued to assist communities ever since. While Road Diets are becoming more common and research shows significant benefits, they remain controversial for many communities.

“It can be difficult for the public to see the value of a Road Diet and how reducing the number of lanes will improve safety without causing excessive delay in their community,” says Willham. “During public engagement, we have to put ourselves in their shoes because the concept is a little counterintuitive. In addressing public concerns, we’re able to share data and provide examples where Road Diets have worked. Once installation is complete and people see it working, their opinions usually change.”

Road Diet Experience

Fifth Avenue South in Fort Dodge and Lincoln Street/Iowa Hwy 14 in Knoxville are two Road Diet projects that Snyder & Associates has worked on.

Prior to converting 5th Avenue South to a three-lane roadway, it was a four-lane facility where speeding was common and the crash rate was higher than average. When the conversion was proposed to the general public, opposition built from local businesses and citizens. Snyder & Associates worked with the city to educate the public on the benefits of the conversion, and the city moved forward with the lane reduction. Since completion in 2014, speeds have dropped through the corridor and the number of crashes have reduced significantly.

The Knoxville project is currently underway.

“In Knoxville, we’re currently assisting the city and DOT with Road Diet concepts and funding applications,” Boes says. “The road [Lincoln Street/Iowa Hwy 14] was in need of repair, so rather than just resurface it and put it back the way it was, the city opted to progressively explore options to improve safety. Collisions involving left-turns in that area were fairly common. By doing a Road Diet in conjunction with resurfacing, the city will maximize its investment and create a safer, more efficient corridor.”

As Perington reflects on his experience, he’s optimistic about the future of Road Diets and looks forward to helping more communities make their streets safer.

“Our [Road Diet] experience dates back to the early 90s, so we’re well-prepared to help assess feasibility, model operations, and address public concerns that are common with Road Diet conversion projects. Safety is paramount, and a Road Diet may not always be the best solution. When it’s not, we’ll help you figure out what is.”

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