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

Measuring and evaluating capacity can be a complex process, particularly for thoroughfares with varying conditions and a lack of uniformity between segments. Street capacity is affected by many 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; and
• 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. 

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 which 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 an arterial street system without widening the street itself (although some transit solutions may require widening the street).

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.  

Credit: CH2M HILL

Transit is especially effective in areas with high density 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 for Small Town Downtowns

The main street for a small town downtown is often a state route, serving significant through traffic in addition to local traffic. For this reason, capacity in small town downtowns is more a function of speed than the number of travel lanes. Characteristics affecting capacity in Small Town Downtowns are:

• The speed differential between "in town" and "outside of town" segments; and
• Pedestrian presence.

Reduce speeds early. State routes outside of small town downtowns, are often higher speed, two-lane highways. Two travel lanes (with turn lane provisions at intersections) usually provide sufficient capacity in town as well. The limiting factor on capacity is the speed reduction required when vehicles approach and travel through the downtown area. The reduction in speed is necessary for the safety and benefit of pedestrians, businesses, and local traffic movement, despite its inconvenience for through-traffic. Advanced signing of speed reductions is essential in controlling speeds of traffic approaching the downtown area. Enforcement of the speed transition areas may be necessary if speed limit violation becomes a problem.  See the Design Speed section of this manual for further discussion.

Create a sense of entry 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 small town and left the higher-speed environment. Intersection and pavement design can also help create a change in environment. Roundabouts, which are a safe, effective and attractive way to improve intersection capacity can help calm traffic upon entry (and exit) from the small town downtown. 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 downtown feel. The small town downtown is first and foremost, the primary commercial area for the residents of the town. Through-traffic should be safely accommodated and indeed supported for the economic benefit of downtown businesses, but it should be clear to through-traffic drivers that they have entered a downtown area with slower speeds, and the potential for stop-and-go traffic, allowing people to cross the street, park and navigate through town safely. Small towns may consider diverting truck traffic if the impacts on the downtown are intolerable. However, the process of identifying and implementing a truck by-pass can be a contentious one. Heavy truck traffic is only appropriate on certain types of thoroughfares and there are significant economic impacts associated with diverting it.

Plan and design for the pedestrian. Preserving the character of small towns is key to making and preserving great streets. Small town downtown 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. 

Resources

PDFs:

• Missouri Practical Design
• Florida Level of Service Table
• Florida DOT Level of Service 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 Controls
• FHWA Roundabouts: An Informational Guide, Chapter 4 - Operations

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)