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Capacity

Article

Symbiotic Credit: css.org/CH2M HILL Great streets and great places look beyond the lanes carrying vehicular traffic to the pedestrian realm and adjacent land uses.

The image at right depicts the symbiotic relationship between motor vehicles, buses, MetroLink, bicyclists, and pedestrians that is necessary to create great streets. 

We must recognize the symbiotic relationship that is necessary for all of these modes to flourish (not simply coexist) in one common environment. 

As Allan Jacobs notes: "It's no big mystery. The best streets are comfortable to walk along with leisure and safety. They are streets for both pedestrians and drivers."

In designing great streets, capacity considerations will influence how much space should be allocated to vehicular traffic, transit, bicyclists, and pedestrians.  

Capacity is a basic measure of the quantity of traffic a road can carry, or more specifically, the maximum sustainable rate at which pedestrians, bicycles or vehicles can be expected to travel across a defined point or segment during a period of time, typically expressed in vehicles per hour or pedestrians per hour.

Measuring and evaluating capacity can be a complex process, particularly for arterial streets with varying conditions and a lack of uniformity between segments. Street capacity is affected by many other design considerations, such as mobility and access. Increasing the number of access points along an arterial, for example, can severely limit capacity while increasing vehicular capacity can negatively impact the mobility of pedestrians. These concepts must all be carefully balanced, for all modes, to ensure great and safe streets.  

Creating great streets begins with a solid understanding of the type of place you hope to create. The appropriate role of capacity for a given roadway is determined by:

  • The degree to which various modes are present
  • Abutting land uses
  • The role of the arterial within the context of the network

A common mistake municipalities make is widening roadways to enhance capacity and improve the flow of vehicular traffic, without considering all its effects. Roadway widening may be appropriate along some mobility-priority corridors. However, for corridors with a significant pedestrian presence, commercial, mixed-use, or residential development, and/or widespread transit use, widening streets to increase capacity is not the preferred solution. Widening streets detracts from the pedestrian experience, jeopardizes pedestrian safety, can displace or limit development, and may discourage transit use. 

Who says we need more lanesWhen determining the appropriate number of vehicular travel lanes for a given corridor, it is important to consider the effects of such widening on pedestrians and adjacent land uses. The Florida Department of Transportation has developed several tables that can be useful for guiding capacity choices. The tables use a set of default values to approximate the amount of traffic a facility can carry based on the number of lanes, median type, number of traffic signals, and desired level of service.

Bus lane capacity
Credit: CH2M HILL

These tables were designed for general planning purposes and are most useful in assessing the overall capacity needs of a facility, as opposed to specific LOS measures such as delay or average travel speed. See the Florida LOS table for more details.  Transit can be an ideal way to add capacity to an arterial street system without widening the street itself (although some transit solutions might require widening the street, too).

Bus lane
Credit: CH2M HILL

Bus service can reduce the number of single-occupancy passenger cars on the street, resulting in better vehicular operations overall.  Dedicated bus lanes and/or bus rapid transit should be considered in arterial corridors for long-range person movement capacity.  

HOV lane
Credit: CH2M HILL

Transit is especially effective in areas with relatively dense land uses that can produce stable and consistent ridership. Arterial corridors with heavy through-traffic should also consider placing a higher priority on bus lanes.

In light of the expanding bus and light rail systems, St. Louis should begin prioritizing transit along the region's arterials to increase capacity and mobility for all modes.  It is important to point out, however, that bus lanes and other transit capacity measures are not without their challenges.  Provision of these measures, particularly at intersections, must be carefully implemented to insure that they do not negatively impact the efficiency or safety of the overall intersection.  See the Intersections section of this guide for more details.

Capacity along Neighborhood Shop Thoroughfares

Primary characteristics affecting capacity for neighborhood shop place types include:

  • Significant pedestrian presence; and
  • Economic development needs.

Depending on the target speed established for a neighborhood shop thoroughfare, a variety of volumes can be accommodated at several different levels of service. The tables below offer some general capacity thresholds, and can serve as an effective guide when considering the appropriate number of lanes to provide for a given street.

LOS and ADT at 30 mph
Credit: CH2M HILL
LOS and ADT at 35 mph
Credit: CH2M HILL

Plan and design for the pedestrian. Preserving the character of neighborhoods is key to making and preserving great streets. Neighborhood shop areas should be pedestrian places, where residents gather and feel welcome. Vehicular traffic should be appropriately calmed and decisions about vehicular capacity should consider the impacts on pedestrian access and mobility. Increasing capacity at intersections often means adding turn lanes, which widens the overall footprint of the intersection and makes it much more difficult for pedestrians to cross safely. Larger intersections also have negative implications for adjacent developments and properties, and the longer walk phase time required for pedestrians to navigate the intersection can increase signal delay. 

Create a sense of place and identity. Design elements, such as narrower thoroughfares with street frontage, on-street parking and decorative signing can help create an environment more supportive of lower speeds by providing clues to drivers that they have entered a neighborhood shop environment. Intersection and pavement design can also help calm traffic and raise driver awareness in the neighborhood shop environment. Roundabouts, which are a safe, effective and attractive way to improve intersection capacity can help calm traffic upon entry (and exit) from the neighborhood shop area. There are cases where the geometric design and space requirements for roundabouts render them impractical, especially for intersections surrounded by existing development. Roundabouts are generally not appropriate at intersections where pedestrian crossing volumes are expected to be high. See the Intersections section of this manual for more details about roundabouts and other intersection treatments.

Maintain the neighborhood feel. The neighborhood shop place type is first and foremost the primary commercial area for the residents of the surrounding neighborhoods. Through-traffic should be safely accommodated and indeed supported for the economic benefit of the local businesses, but it should be clear to through-traffic drivers that they have entered a place with slower speeds and the potential for stop-and-go traffic, allowing people to cross the street, park and navigate through town safely. 

Locate bus stops appropriately. (mid-block, nearside, or farside). Transit should be encouraged and prioritized along neighborhood shop thoroughfares. Bus service will usually be the most common form of transit reaching these areas. For bus stops, the most appropriate location typically depends on site-specific characteristics.  The tables at right and below describe the advantages and disadvantages of nearside, farside, and midblock bus stop locations. Bus stop spacing is also important in prioritizing transit.  The Federal Transit Administration article Stops, Spacing, Location, and Design, provides additional information on appropriate spacing and location considerations, with a focus on Bus Rapid Transit.

Prioritizing transit requires more than just transit-specific considerations. To truly prioritize transit, we must also prioritize pedestrian travel along the street.  Transit inherently produces significant pedestrian traffic along the street. If safe and efficient pedestrian accommodations are not provided, transit will not be viewed as a desirable travel choice (see the Transit section of this guide for more information).  As with all pedestrian accommodations, we must ensure that these elements provide safe and efficient service to persons with disabilities, as discussed in the Universal Design section of this guide.

Economic development is often a high priority in neighborhood shop areas. Many of the commercial businesses in these areas serve the neighboring residential population. Coffee shops, news stands, convenience stores, restaurants, banks, and other businesses will all benefit from a great neighborhood street traveling by their storefront.  

One of the important automobile provisions necessary to serve these businesses is parking. The Parking section has a more detailed discussion of specific neighborhood shop applications. Parking is an important design element that has immediate economic impact on adjacent businesses, but also an impact on street capacity. For this reason, jurisdictions should encourage on-street parking along mixed-use streets and not be tempted to eliminate it in order to expand capacity for vehicular traffic. While eliminating on-street parking would potentially provide an additional travel lane, on-street parking serves as an important buffer between vehicular traffic and pedestrians, improving safety and the overall quality of the pedestrian experience, while serving business interests. It also calms through-traffic by visually narrowing the roadway.  

Resources

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Other References:

  • AASHTO Green Book, Chapter 7, pg. 470 (2004 Edition)
  • ITE Traffic Engineering Handbook, Chapter 4 (1999 Edition)
  • TRB Highway Capacity Manual, Chapters 10, 15, and 16 (2000 Edition)