Capacity

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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 thoroughfare 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 thoroughfares with varying conditions and a lack of uniformity between segments. Thoroughfare capacity is affected by many other design considerations, such as mobility and access. Increasing the number of access points along a thoroughfare, 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 thoroughfare is determined by:

• The degree to which various modes are present;
• Abutting land uses; and
• The role of the thoroughfare within the context of the network.

A common mistake municipalities make is widening thoroughfares to enhance capacity and improve the flow of vehicular traffic, without considering all its effects. Thoroughfare 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 thoroughfares to increase capacity is not the preferred solution. Widening thoroughfares detracts from the pedestrian experience, jeopardizes pedestrian safety, can displace or limit development, and may discourage 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 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 a street system without widening the street itself (although some transit solutions might require widening the street, too).

Credit: CH2M HILL

Bus service can reduce the number of single-occupancy passenger cars on the street, resulting in better vehicular operations overall. Credit: CH2M HILL Dedicated bus lanes and/or bus rapid transit could be considered in thoroughfare corridors for long-range person movement capacity.

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

As part of expanding the regional bus and light rail systems, St. Louis is beginning to evaluate the prioritization of 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 prioritization 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 along Downtown Main Streets

Primary characteristics affecting capacity along Downtown Main Streets are:

• 

  Credit: EW Gateway
  Economic development needs;
• Transit and pedestrian presence; and
• Access requirements.

Trade capacity for economic and safety benefits. Economic development is often a high priority in downtown main street corridors. The potential for congested conditions in these areas resulting from lower capacity may be acceptable if reducing capacity helps to stabilize commerce and create growth in the downtown, as shown at right. Parking is an important downtown main street element that has an immediate economic impact on adjacent businesses, but also an impact on thoroughfare capacity.

For this reason and others, jurisdictions should encourage on-street parking along downtown main streets and not be tempted to eliminate it in order to expand capacity for vehicular traffic. While eliminating on-street parking would potentially allow 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. It also calms through-traffic by visually narrowing the thoroughfare

The capacity of the larger transportation network is an important consideration when determining the appropriate capacity for a thoroughfare running through a downtown area. Additional capacity in the surrounding network may further substantiate capacity reductions along the downtown main street corridor, as through-traffic would have alternate routes. Referred to as a Road Diet, this approach can encourage development consistent with the goals of a downtown area. For more information about road diets, see Streets . The relationship between the section of roadway in the downtown, the broader corridor, and the regional network as a whole will also inform decisions about capacity. For example, reducing a thoroughfare from four to three lanes may cause congestion at the area of transition if the greater thoroughfare remains a 4-lane facility outside the downtown. The reduction should be carefully planned and located to minimize the effect of the "bottleneck" on the overall thoroughfare.

Depending on the target speed established for a downtown main 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 thoroughfare.

Credit: CH2M HILL

Credit: CH2M HILL

The ITE guide Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities also provides some good general parameters on capacity for downtown main streets (see Tables 6.2 and 6.3). The "Traveled Way" section of those tables provides minimum and maximum daily volume ranges for a variety of facility types.

Credit: CH2M HILL Keep intersections small. The significant pedestrian presence along downtown main street corridors also has implications for capacity. 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. Resist the temptation to enlargen intersections in downtown areas. Congestion is more tolerable and it keeps downtowns safe for other modes.

See the Intersections section of this manual for more details about intersection treatments.

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)
• Virginia Multi-Modal Planning Study
• Multimodal Level of Service Presentation
• HCM Perspective on Arterial Performance Measures
• FHWA Bicycle and Pedestrian Course
• Chicago Corridor Transit Feasibility Study
• Iowa Road Diet Study

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
• Highway Capacity Manual Applications Guidebook
• TDOT Transportation Management Office - HELP Program

Other References

(not currently available in electronic format)

• 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)