Bicycle Safety Guide and Countermeasure Selection System
"Before" picture from a typical four-lane to three-lane conversion. Source: Federal Highway Administration
Some roads have more travel lanes than necessary, and the width of the excess lanes could be freed up for other uses. Space may be better used for bicycle lanes, parking, or wider pedestrian buffers or sidewalks (with curb realignment). A traffic analysis should be conducted to determine whether a lane reduction is feasible, (i.e., 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 ADT 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, where volumes are low on a three-lane road (one lane in each direction with a TWLTL) can be converted to two.
Extra roadway space can be reallocated for other roadway users to improve safety, comfort, and convenience (see list above). 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:
Reducing lane numbers remedies excess capacity situations. More space for bicyclists, pedestrians, and parking is provided. Reducing the apparent width of the road provides median refuge and improves safety for all street users. It improves social interaction and enhances livability of the street.
Lane reductions (i.e., road diets) optimize street space to benefit all users. Lane reductions help improve safety and comfort for pedestrian as well as bicyclists. Reducing the number of lanes on a multilane roadway can help improve sight distances for left-turning vehicles and create space for bicycle, transit, and/or parking lanes.
There are a number of factors to consider when determining the appropriateness of a lane reduction. These include:
The cost for restriping a mile of four-lane street to one lane in each direction plus a TWLTL and bike lanes is about $5,000 to $20,000 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 is little or no cost for space reallocations accomplished through new striping.
Authors and Acknowledgements