Lateral Shifts and Chicanes

Image Source: NACTO

DEFINITION AND DESCRIPTION

A lateral shift realigns a straight street by shifting traffic in one direction (i.e., to the left or right). Chicanes are a series of alternating lateral shifts that create multiple changes in direction along a roadway segment. Both Lateral Shifts and Chicanes use features such as pavement markings, planting strips, on-street parking, and other features create a sequence of horizontal curves intended to slow vehicle speeds, improve driver attentiveness and reduce crash risk. Curves or shifts can be gentle for modest speed reduction, or more restrictive for greater speed management. Shifts in travel-ways can be created by moving parking from one side of the street to the other (if there is only space for one side of parking), or by building landscaped or hardscaped islands.

Implementation Categories

CONSIDERATIONS

• Lateral Shifts and Chicanes encourage motorists to slow down when negotiating curves.
• Landscaped islands can be aesthetically pleasing. Consider an MOA with the local government for long-term maintenance of landscaping.
• Roadway drainage needs should be considered.
• May be difficult for large vehicles to navigate if not designed properly. Should not be used in industrial areas, or routes with significant truck, bus or emergency response activity.
• Using median islands to shift traffic reduces the likelihood a motorist will veer into the path of opposing traffic.
• Mountable curbs should be used on edge and median islands to improve accessibility for larger vehicles.
• Most effective on roadways with posted speed limits ≤ 35 MPH. On higher-speed roads, lateral shifts should be gradual and spread over longer distances.
• If used on multi-lane roadways, consider that vehicles may drift into the adjacent lane; provide adequate width or use channelizing elements.
• Installation could be coordinated with the resurfacing schedule.
• Temporary “quick-build” materials (bollards, striping) may be used to test effectiveness before committing to permanent installation.
• Assess conditions and include accommodations for bicyclists and pedestrians as appropriate. Avoid “pinch points” for bicyclists.
• Include signing and pavement markings (e.g., chevrons, arrows, edge lines) to clearly guide drivers through the alignment.
• Evaluate sight distance through the chicanes to ensure safe operation.

APPLICATION

• Use in speed transition zones (e.g., where roadway context changes) and roadways with medium or high levels of pedestrian activity.
• May be appropriate for local, collector, and minor arterial streets requiring traffic calming. Can be implemented on one-way or two-way roadway segments.
• Raised islands using mountable curbs are recommended where feasible; otherwise, delineation with pavement markings is acceptable. May use pavement markings, medians, bollards, planters, on-street parking shifts, landscaping and/or streetscape elements.
• When curves are used, select radii that match the appropriate speed for the context. Refer to AASHTO Greenbook Table 3-13, Minimum Radii and Superelevation for Low-Speed Streets in Urban Areas.
• Edge line taper lengths (L) should conform to the MUTCD. L = WS²/60, where L is the taper length (ft), W is the lateral offset (ft), and S is the design speed (mph). Traffic calming effects are greatest when lateral offset, W, is at least one full lane width.
• Raised curb radii is typically 2 ft.
• A return angle of around 45° is typically used for raised islands.
• The length of curb extensions or edge islands should extend at least 20 feet long, but can be longer depending on the use desired within the extension (e.g., stormwater management, restricting parking)
• In cases where drainage is a concern, edge islands with a 1-2 ft gap from the curb are recommended.

example

Image Source: Delaware DOT

Complementary SSA Strategies

• Roadway Reconfiguration
• Successively Smaller Curves