By Chris McCahill
A study released by Johns Hopkins last November gained widespread attention for demonstrating that 9-foot lanes are often safer than wider lanes. The researchers note, however, that most state DOTs set minimum lane widths between 10 and 12 feet and require design exceptions for anything narrower. Even in Vermont, where 9-foot lanes are allowed, the researchers found they have not been implemented. Therefore, paving the way to narrow lanes means understanding all the factors that make them challenging in the first place.
Before getting into those factors, what exactly did the Johns Hopkins study find? The researchers analyzed crash rates on more than 1,100 road segments across seven cities. All else being equal, they found 9-foot lanes are just as safe as 10- and 11-foot lanes, and they are safer than 12-foot lanes. At speeds below 30 mph, there is no significant difference among lane widths, but at speeds between 30 and 35 mph, 9-foot lanes are the safest.
There are many reasons 9-foot lanes rarely make it through the design process. In Vermont, for example, the Johns Hopkins study found:
Liability was cited as the main concern for opting for wider than 9-foot lanes, but also weather especially in winter and the maintenance costs associated with snowfalls make narrower lanes challenging for states such as Vermont.
Some common obstacles are outlined below, along with steps that could be taken to make narrow lanes more viable in the right contexts.
1. Local design guidelines.
Although some states may have loosened their guidelines, a 2010 study published in ITE Journal noted that 24 states established 11-foot minimum lane widths and another six states set 12-foot minimums. The Johns Hopkins study outlines some of the justifications that agencies give for allowing narrower lanes. Those generally include lowering traffic speeds, improving safety, increasing bike and pedestrian use, and reducing construction and maintenance costs. However, the design exception and approval process can be onerous at times:
Some states don’t have specific criteria and review lane width exception projects case-by-case, based on factors such as funding, impacts on property, impacts to the environment, speed, traffic volume, and modal accommodation. Other states consider lane width reduction projects mostly based on transportation-related criteria such as roadway classification, traffic volume (AADT), and operating speed.
2. Legal concerns.
While the standards outlined in AASHTO’s Green Book are only required on the National Highway System (Title 23, Part 625), road designers often lean on those standards to mitigate their legal liability on other roadways. Researchers from the Texas Transportation Institute wrote in 1986:
Applications of the design criteria and guidelines contained in the Green Book will provide safer, more efficient, and more comfortable roadways. Hence, the safer roadways will help to reduce traffic accidents, which in turn will help to minimize tort-related lawsuits resulting from accidents. Compliance with the Green Book is an effective method of reducing highway tort liability.
That mentality holds true for many agencies today.
3. Traffic capacity.
Common highway design guidelines translate wider lanes to higher road capacity, which is often a top priority for road designers. As the Johns Hopkins study explains, each 1-foot reduction in width lowers vehicle capacity by 3.3% at signalized intersections, according to the Highway Capacity Manual. This is because people tend to drive slower in narrower lanes, which translates to fewer vehicles per hour (aka, “flow rate”).
4. Trucks and other large vehicles.
Roads on the national freight network require at least one 12-foot lane. States may apply this standard to other designated freight routes. As an extreme example, an Illinois statute requires all state routes to consider a large semi-truck as the default design vehicle, forcing the Chicago DOT and Illinois DOT to reach an agreement allowing 10-foot lanes.
A standard bus can also be upwards of 10 feet wide, with its side mirrors, which means transit agencies often want lanes that are at least 11 feet wide, especially when two buses could pass each other in opposite directions. One study analyzed bus crashes in Austin, looking at things like sideswipe, fixed object, and mirror-to-mirror incidents. It found every one-foot decrease in lane width increased crash risk by around 10%.
5. Emergency access.
The International Fire Code requires fire access roads to be at least 20 feet wide (once a topic of dispute in Celebration, Florida, among other places). It also requires a minimum unobstructed width of 26 feet near fire hydrants or near buildings over 30 feet tall. There are exceptions for certain types of buildings with sprinkler systems and enclosed stairways that access the roof.
6. Winter maintenance, parking, and other obstacles.
As noted in Vermont, extra road width can be especially helpful during snowstorms. Wider lanes can also allow more space between traveling vehicles and parked cars or people on bicycles.
Getting to narrower lanes…
Allowing narrower lanes in design guidelines, especially without burdensome exception polices, is one important step to improving safety on urban roads. But that is clearly not enough. Many road designers still need assurance that their design decisions will hold up in court, and they require flexibility to explore innovative solutions. The Johns Hopkins study offers some encouragement, but it does not translate to mainstream acceptance.
When it comes to concerns about wider vehicles and other obstacles, road designers may want to start by experimenting with narrower lane markings, while preserving the unobstructed road width, to test their effects on safety. The additional space sometimes can provide additional buffers for parked cars or bike lanes. Several studies even suggest wider painted stripes can help lower speeds.
Given the inevitable need for somewhat wider lanes to accommodate larger trucks and buses, city planners may want to consider how they designate freight and transit corridors and limit the number of local roads impacted.
Beyond that, road designers may need to have candid conversations with local emergency response officials. This could lead to short term road installations that let fire response crews experiment with their vehicles, revised fire codes (like in Baltimore), new building codes that stress fire prevention and access, or the acquisition of smaller emergency response vehicles. A report prepared for Los Angeles County in 2013 outlines important considerations.