Capacity

Article

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.

Capacity considerations will influence how much space should be allocated to vehicular traffic, transit, bicyclists, and pedestrians. The image at right depicts the symbiotic relationship between motor vehicles, buses, MetroLink, bicyclists, and pedestrians that is necessary to create great streets. 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."

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, including 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. Great streets balance these needs, for all modes.

Great streets begin 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; and
• The role of the arterial within the context of the network.

A common mistake municipalities make is widening roadways to enhance vehicular 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 significant transit use, widening streets to increase capacity is not the preferred solution. Widening streets detracts from the pedestrian experience and may jeopardize pedestrian safety. Thoroughfare widening can also displace or limit economic development, and may discourage bicycle and transit use.

When 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.

Credit: CH2M HILL

These tables were designed for general planning purposes and are most useful in assessing the overall vehicular 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 certain transit solutions involve widening the street). 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 could be considered in arterial corridors for long-range person movement capacity.

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 could consider placing a higher priority on bus lanes, such as the one shown at right.

In light of the expanding bus and light rail systems, St. Louis communities can prioritize transit along the region's thoroughfares 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 ensure 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 in Mixed-Use Districts

Primary characteristics affecting capacity for mixed-use streets include:

• Transit presence;
• Significant pedestrian presence; and
• Economic development needs.

Depending on the target speed established for a mixed-use street, a variety of volumes can be serviced at several different levels. 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.

Credit: CH2M HILL

Credit: CH2M HILL

Mixed-used districts are ideal settings for bus and light rail lines, because they typically include a variety of moderate to high-density mixed land uses which are necessary to support transit. For this reason, Transit-Oriented Development, is designed to include a mix of uses that supports transit use.

A variety of planning and design tools are available to help mixed-use districts prioritize transit:

Credit: CH2M HILL

Transit-only lanes. Traditionally, buses use the general vehicular travel lanes. Pull-off lanes, or pull-outs, at bus stops, can make it difficult for buses to reenter the stream of traffic and decrease the efficiency of transit use.

Transit-only lanes can help place a higher priority on buses and other transit service. These lanes provide dedicated space on the street for buses (and sometimes bicyclists or high-occupancy vehicles) and can help make transit a more efficient, viable, and attractive choice.

Transit-only lanes can be located on the outside of the travel way (near the shoulder) or on the inside of the travel way (in the median). Transit-only lanes may be in use throughout the day, or during peak periods only.

Credit: CH2M HILL

Peak period prioritization is especially useful in areas with an extremely heavy peak period. The image at right shows an example of rush hour transit lanes that also serve bicyclists and high-occupancy vehicles (HOVs). An article from the Federal Transit Administration provides more information about Bus Lanes.

The degree to which transit lanes are appropriate is influenced by the following considerations:

• Transit lanes occupy space that would otherwise be allocated to either vehicular traffic, the pedestrian realm, or storefront businesses.
• Ridership levels must be high to achieve the types of capacity benefits that are possible with transit lanes. Low ridership makes transit lanes a less viable alternative.
• Transit lanes require special planning and design at intersections. Care must be taken to manage the conflict between transit-only lanes and right-turning vehicles, which essentially requires a lane transition. These transitions must ensure that all users are able to safely and efficiently execute turning maneuvers. Pavement striping, clear signing, and taper lengths are important elements in effectively managing these transitions.

Transit bypass lanes, or "queue jumpers" are used to prioritize buses at intersections, allowing them to bypass congested queues forming in the vehicular travel lanes. Roadway designers should be aware of the potential conflict between transit bypass lanes and right-turning vehicles at intersections.

Source: ITE CSS Guide

Appropriate location of bus stops (mid-block, nearside, or farside). 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.

Source: ITE CSS Guide

Traffic Signal Priority is another way to prioritize transit along the street. Special treatments and traffic signal technologies are available to detect and prioritize transit vehicles at signalized intersections. See the Federal Transit Administration's article on Signal Priority for more information.

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 mixed-use corridors. Many of the commercial businesses in mixed-use areas serve commuters traveling through the corridor. Coffee shops, news stands, convenience stores, restaurants, banks, and other businesses will all benefit from a great mixed-use street traveling by their store-front.

One of the important automobile provisions necessary to serve these businesses is parking. The Parking section has a more detailed discussion of specific mixed-use applications. Parking is an important mixed-use 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

PDFs:

• Florida DOT LOS Table
• Florida DOT LOS Handbook
• ITE Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities (See Chapter 7)

Links:

• FHWA Flexibility in Highway Design, Chapter 4 - Design Controls
• Massachusetts Design Guide, Chapter 3 - Design Control
• FHWA Roundabouts: An Informational Guide, Chapter 4 - Operations

Other Resources:

• 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)