Segments

Warning

This section is a work in progress. It has been ported from an older document and is very many years out of date.

Note

We may rename “segments” to something else, e.g. “slices.”

Notes on street segments. See Issue #7 for discussion around more street segments.

Automobile / Drive lanes

Typical drive lane

Lane widths typically fall between 2.7m (9ft) and 3.6m (12ft). For urban contexts where Streetmix is most useful, the 3.0m (10ft) lane width is most ideal. Some guides allow for 3.3m (11ft) lanes, but we will follow Jeff Speck’s lead (and other progressive planners) in setting 10ft widths as typical.

There may be outlying scenarios where wider lanes are needed, but these should never be considered the default: a 12-ft lane may be desirable to provide passing clearances for large commercial vehicles on two-lane highways, and widths less than 10-ft can be more desirable for urban and residential areas where pedestrian crossings exist and lower traffic speeds are needed.

When there is more than one lane in a direction, sometimes the outside drive lane can be wider than inside drive lanes, and even increased beyond the 12-ft maximum range, to accommodate larger vehicles (such as trucks) as well as bicycles (see also sharrow). Note that when space allows, a dedicated bike lane is always preferred and safer than a sharrow.

need source tying lane width to maximum car speed and safety

Design considerations

  • Generally, any lane improvement that increases traffic flow (and speed) will correlate to an increase in pedestrian injuries and deaths, and reduce walkability. For instance:

Typical turn lanes

Turn lanes are typically provided on major streets to provide a refuge for cars turning into cross-streets, side streets, or sometimes even into a parking lot. At intersections, space for turn lanes are often created by removing a parking lane, or by converting a through lane to a turn lane (and dual-purpose through/turn lanes exist as well). If the street is particularly wide, a median can be provided between the directional lanes, and the turn lane occupies the space where the median was when needed. On streets with a lot of possible turns, using a continuous center turn lane is common.

Center turn lane

A center turn lane is a special type of turn lane that is continuous along the street, allowing refuge for turns along the entire street and not just where a specific turn lane area is provided.

Adding a center turn lane is a good “road diet” tool for turning dangerous four-lane roads into a three-lane road (two lanes in either direction, with the center turn lane as the third lane). However, when street widths are fixed, it’s usually not a good idea to take away some other kind of amenity (like parking lanes or bike lanes) to provide the turn lane. (for more information, see Walkable City “A Road Too Far”)

Parallel parking lanes

  • Subtypes: inbound, outbound
  • Default width: 8 ft (Source: Complete Streets Chicago 3.2.1 “Cross Section Assemblage” (2013))
  • Minimum width: 7 ft (Source: Complete Streets Chicago 3.2.1 “Cross Section Assemblage” (2013))
  • Maximum width: 10 ft
  • Markings: unmarked, marked with “T”

On-street automobile parking lanes are common and have been generally included wherever there is room. In residential areas, the minimum width is 7 ft and are usually unmarked. On commercial streets the width is usually minimum of 8 ft and may be marked so that the number of parking spaces fixed to a specific amount, and can be tied to parking meters.

Some municipal guidelines look at drive lanes and parking lanes together, especially when parking lanes are unmarked. For instance, the Las Vegas Unified Development Code designs residential streets to be 17’-18’ (so that it reasonably breaks down into 10’ drive, + 7’-8’ parking). Note that the Chicago Complete Streets Plan, cited above, requires an 18’ minimum - this creates a 11’ drive lane with a 7’ parking lane in a residential zone, which - given that 10’ is plenty particularly in residential streets where speeds should not be encouraged to go above 20-25mph - we should not accept as ideal.

Other types of parking lanes

Guidelines and helpful plan diagrams for these exist in certain municipalities, but I have to remember which ones and then track them down.

Perpendicular

  • Default width: 18-20 ft
  • Minimum width: 14-18 ft
    • A car’s length is usually more than 14 ft but some municipalities allow the front of cars to overhang curbs.
  • Maximum width: 22 ft

A situation where cars are allowed to park perpendicular to the curb. Mostly seen on extremely wide commercial corridors.

Angled

Like perpendicular, but angled parking takes up less width. The angle of the parking space will influence its width.

Bike facilities

Generally, for Streetmix, bike lane and bike facilities will adopt the NACTO Urban Bikeway Design Guide, which provides in-depth discussion on the different types of bike paths that are available, based on guidelines set out in the AASHTO Guide for the Development of Bicycle Facilities (1999). Note that our print copy is the April 2011, but we should be using the second edition of the NACTO guide released in 2012.

Typical bike lane

  • Subtypes: Inbound, outbound
  • Default width: 6 ft (Source: NACTO text, diagram)
    • Note that many practitioners (such as Walkable City’s Jeff Speck) follow AASHTO guidelines for default bike lane width, which is 5 ft. NACTO allows for it but prefers to advocate for 6 ft wherever possible.
  • Minimum width: 3 ft (Source: see above)
  • Maximum width: 8 ft (Source: ?)

Bike lanes designate an exclusive space for bicyclists through the use of pavement markings and signage, and are usually located adjacent to motor vehicle travel lanes and flows in the same direction as motor vehicle traffic. Bike lanes are typically on the right side of the street, between the adjacent travel lane and curb or parking lane, although in some countries the preferred location is between the parking lane and the curb. For other buffer types, see below.

Bicycles are a desirable alternate form of transportation to the automobile because it is a form of personal vehicle that has a lot of benefits, which we won’t describe here. Providing bike lanes allows planners to be equitable to different forms of transportation while reducing a carbon footprint and also allows for more passengers to occupy a road, and separating the lanes helps bicyclists be safe and comfortable riding on the street. It also reminds car drivers that bicyclists are present. Bicyclists may leave the bike lane to pass other bicyclists, make left turns, avoid obstacles or debris, and avoid other conflicts with other users of the street.

Shared lane markings, or “sharrows” (bike lane + drive lane)

  • Subtypes: Inbound, outbound
  • Default width: 14 ft (4.2m) (Source: AASHTO Green Book 2011, page 100; Complete Streets Chicago 3.2.1 “Cross Section Assemblage” (2013))
  • Minimum width: 12 ft (3.6m) (Source: AASHTO Green Book 2011, page 316)
    • AASHTO’s actual suggestion of 12-13 ft (3.6-3.9m) describes a situation where an outside drive lane is slightly wider than an inside drive lane only to allow for bicycles. Presumably, drivers are allowed or expected to pass bicyclists on the inside lane.
  • Maximum width: 14 ft (at 15 ft, you may as well put in a 10’ lane and a 5’ bike lane.)

Sharrows are a strategy where additional road space is available, and you want to get a bike lane in there somehow, but you can’t really stripe it without giving both the drive lane and bike lane less than the standard width, so it is combined. This is a “shared lane” scenario.

In a California study, researchers found that drivers tend to prefer real bike lanes to keep bikers separate from the cars, and dislike sharrows. Bikers, on the other hand, prefer sharrows to nothing at all. (Source: Do All Roadway Users Want the Same Things? Results from a Roadway Design Survey of Pedestrians, Drivers, Bicyclists, and Transit Users in the Bay Area by Rebecca L. Sanders and Jill Cooper, 2013)

Cycletrack (bike lane + median or buffer)

  • Subtypes: One-way, two-way
  • Painted buffer. 2 solid white lines with diagonal hatching. 2 feet minimum because it would be impractical to paint a buffer area less than this. 3-feet buffer seems to be a pretty appropriate starting default. Buffer can be included with the bike lane as the total “bike lane width” (so a 2-ft buffer + 5-ft bike lane, or 3-ft buffer + 4-ft bike lane = 7-ft bike lane).
    • Source: NACTO Urban Bikeway Design Guide (April 2011) pp 20-22

Other types of bike lanes

For discussion around other types of bike lane placements, such as left-hand side bike lanes, or contra-flow bike lanes (both of which are permissible in Streetmix) see the NACTO Urban Bikeway Design Guide.

Bike parking

Other road infrastructure

Medians (divider only)

  • Subtypes: Curb only, planted

1.2m - 24m (4ft - 80ft) or more - AASHTO Green Book p341. Does this apply only to highways? pp341-343 has information

Pedestrian median

A median that is at least 6’ wide is good for pedestrian crossings. When designed as “refuge islands” on two-way streets, it allows pedestrians to cross one direction of traffic at a time in a safer way. While 6’ is the minimum needed to accommodate the length of a bicycle or a person pushing a stroller, the preferred width of a refuge would be between 8 to 10 feet, and even wider where there are more pedestrians than usual. Trees, landscaping, signage, lights, or other “street furniture” could also be installed on the median to make the refuge obvious to drivers.

Source: Complete Streets Chicago pg 106.

Shoulders and gutters

Not presently planned for Streetmix.

Public Transportation

Trolleys, streetcars, light rail

  • Subtypes: Different car types; inbound, outbound; separated infrastructure or in drive lane
  • Default width: 10 ft (same as drive lane)
  • Minimum width: 10 ft
    • According to streetcar.org, the widths of most streetcars are between 8’-0” to 9’-0”. In the absence (so far) of guidelines defining streetcar width, we can assume that 10’ should be the minimum and can remain the default.
  • Maximum width: none

Buses

Trains

Sidewalks (pedestrian zones)

Sidewalk

Sidewalks are essential for pedestrian activity, and when they are made as safe and welcoming as possible for people, it helps to create lively, vibrant streets.

  • 2.4m (8-ft) minimum (but total including buffer)
  • residential areas 4ft-8ft (1.2-2.4m)
  • buffers: 2ft

AASHTO Green Book pp361-362

Sidewalk border

SIDEWALK “BORDER” (aka planting strip, buffer, setc)
minimum 2ft (0.6m)

Sidewalk infrastructure (furnishing zones)

Street Trees

Generic types

Probably no need to be specific about species, which is something landscape architects do all the time.

  • Palm tree
  • Cherry tree (flowering)

Here’s some really good examples for San Francisco.

Miscellaneous