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.

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. 

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.

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 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.  

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.

Primary characteristics affecting capacity for residential place types include:

• Significant pedestrian and bicycle presence; and
• Transit presence

Depending on the target speed established for these thoroughfares, 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.

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 residential area and left the higher-speed environment. Intersection and pavement design can also help create a change in environment.

These places are teeming with pedestrian activity, and should therefore provide environments that are safe, efficient, and appealing for pedestrian travel. To do so, planners, designers, and local leaders should focus on the following pedestrian measures:

• Provide sidewalks that are continuous and wide enough to provide the desired types of pedestrian services
• Maximize the amount of lateral/horizontal separation between pedestrians and motorized traffic
• Keep motorized speeds on the street as low as practicable
• Provide frequent, reliable, and easily accessible bus service

Commuting to work or riding to school via bicycle is an increasingly popular mode of transportation. Such travel should be encouraged; it is environmentally responsible, reduces vehicular congestion, and is economically inexpensive for users. Bicycle trips begin at the residential neighborhood level, so it is important that these thoroughfares prioritize bicycling. The following measures can be taken to enhance bicycle travel in these civic and educational corridors:

• Maximize the width of the outside travel lane and provide bike lane striping that is clear and visible, as shown at right. In areas where transit lanes are provided for buses, bike lanes can be well accommodated due to the lower volume experienced in those lanes.
• Route trucks and heavy vehicles to other truck designated routes to minimize conflicts with bicyclists.
• Keep motorized speeds on the street as low as practicable.
• When drainage grates are required, make sure they are visible and "bicycle friendly."

Residents who choose transit improve regional capacity. These corridors are typically not major transit routes. The residents surrounding them, however, are ideal candidates for transit use as commuting options to work and school. To encourage such a choice, it is important to provide efficient and attractive connections to the transit system from these areas. Every resident that chooses to utilize the transit network is one less single occupant vehicle (SOV) on the road, thus improving the overall capacity of our regional network. The pedestrian and bicycle considerations described above are part of this provision because they facilitate efficient movement of local residents to transit connections. Additionally, these corridors should consider creative ways to encourage transit use. Way-finding signs that guide pedestrians to nearby transit connections are one such measure.

When transit routes do exist on residential thoroughfares, prioritizing this mode is essential. A variety of measures are available to do so:

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. 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 front yards of local residential parcels.
• 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.

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, with a focus on bus rapid transit (BRT).

Traffic Signal Priority is another way to prioritize transit along the thoroughfare. Here again, traffic signals are less common along residential thoroughfares.  Where they do exist or are needed, though, 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.

Resources

PDFs:

• Florida DOT 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 Control
• 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)