Traffic Lane Narrowing or Reduction
Lane narrowing can be achieved in several different ways depending on the type and scope of a project. During all projects there are opportunities to reduce lane widths to the recommended minimums (See AASHTO "Green Book" for further information):
- 9 foot lanes on rural roadways
- 10 feet for most vehicular travel lanes
- 10 feet for turn lanes
- 11 feet for lanes to accommodate large volumes of trucks, buses, or larger vehicles (typically where volumes of large vehicles are greater than eight percent)
To determine if a lane reduction (road diet) is feasible, a traffic analysis should be conducted to see if vehicle capacity exceeds existing and projected volumes (many roadways were built without such analysis and in other cases traffic volumes have receded over time). The most common road diet configuration involves converting a four lane road to three lanes, with one travel lane in each direction and a center two-way left-turn lane (TWLTL), often supplemented with painted, textured, or raised center islands. Left-turning drivers can exit the traffic stream and wait in the TWLTL, while through traffic can maintain a fairly constant speed. Four to three lane conversions should be considered for roadways with documented safety concerns, moderate volumes (less than 15,000 average daily traffic, up to 25,000 average daily traffic in special cases), and along priority bicycle and walking routes.
There are many opportunities to perform road diets, particularly on roadways with wider cross sections, one-way streets (which may have excess capacity), and although not as common, a three lane road (one lane in each direction with a TWLTL) can be converted to two in areas where volumes are low.
Extra roadway space from lane narrowing or reduction can be reallocated for other roadway users to improve safety, comfort, and convenience. Reconstruction projects may allow for curb lines to be moved to narrow the roadway. With the additional space created from restriping or reconstruction, space can be redistributed for the following uses:
- Bicycle lanes or cycle tracks, parking lanes, or transit lanes
- Widened sidewalks, landscaped buffers with street trees, and curb extensions at crossings where on-street parking is present
For more information, check out FHWA's Road Diet Informational Guide (2014).
Lane narrowing and reductions optimize street space to benefit all users. Narrowing lanes or reducing the number of lanes on a multilane roadway can help accomplish the following:
- Improve safety and comfort for bicyclists and pedestrians
- Reduce pedestrian exposure to motor vehicle traffic
- Reduce vehicle speeds, and the potential for rear end collisions
- Improve sight distances for left-turning vehicles
- Provide space for widening sidewalks and installing curb extensions, which also reduces crossing distances and times
- Create space for bicycle, transit, and/or parking lanes
- Create a buffer between motor vehicle traffic and pedestrians (from reclaimed pavement for other uses)
- Road narrowing projects should always consider the surrounding land uses, parking turnover, vehicular speeds, and the volumes and types of traffic (including pedestrians, bicyclists, transit, commercial, emergency response, and heavy vehicles)
- Evaluate whether narrowing may encourage traffic to divert to local neighborhood streets.
- On roadways with excess vehicle capacity, a reduction in the number of travel lanes may be feasible.
There are a number of factors to consider when determining the appropriateness of a lane reduction. These include:
- The volumes and types of traffic, left turn movements, multimodal crash data, roadway widths, sight distance, and the number of driveways.
- Determine if and how alternative routes will be impacted by a lane reduction.
- Lane reductions on higher volume roadways (more than 20,000 ADT) should be studied carefully to ensure that traffic controls and access are appropriate for larger volumes of traffic.
- Estimate the level of service (LOS) for all roadway users, and understand that roadway treatments that result in a lower LOS for vehicles can increase LOS for transit riders, bicyclists, and pedestrians, especially those with disabilities, the elderly and young children. Other factors may include the importance a particular street plays in the pedestrian or bicycle network, and the relationship between creating more livable streets and supporting economic development.
- Consider designs that incorporate raised medians and left-turn bays to help eliminate the potential for TWLTL to be used as acceleration lanes by some motorists.
- On constrained corridors that are major bicycle or transit routes and where vehicle capacity warrants the existing lane configuration, consider removing on-street parking to provide dedicated bicycle and/or transit facilities. Parking removal should be considered on a case-by-case basis using engineering judgment and community input.
- In its guidance memo on road diets, the FHWA recommends consideration of improvements to turn lanes, signing, pavement markings, traffic control devices, transit stops, and bicycle and pedestrian facilities. They also note the classic four-to-three-lane road diet is very compatible with single-lane roundabouts.
- Consider documenting before-and-after studies of the conversion for safety and traffic flow improvements.
Simply adding striped shoulders or on-street bike lanes can cost as little as .15-.20 per linear foot, or approximately $750 to $1000 per mile. Restriping can cost between $5,000 and $30,000 per mile, depending on how many lanes must be removed, and whether bike lanes are added.
The cost for restriping a four-lane street to one lane in each direction plus a two-way left-turn lane with bicycle lanes is about $25,000 to $40,000 per mile, depending on the amount of lane lines that need to be repainted. The estimated cost of extending sidewalks or building a raised median is much higher and can cost $100,000 per mile or more. If a reconfiguration is done after repaving or with an overlay, and curbs do not need to be changed, there may be no additional costs for the reconfiguration or pavement markings.
More detailed cost information can be found here.