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 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.
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.
Capacity for Commercial/Service Corridors
In the St. Louis region, our commercial/service corridors are often the most traveled arterial streets: Olive Boulevard, Watson Road, Gravois Road, Manchester Road, New Halls Ferry Road, Lindbergh Boulevard are examples of the many facilities that serve as conduits for high volumes of vehicular traffic, with little provision for other modes of travel. The focus on their vehicular capacity has allowed these thoroughfares to evolve into very wide roadways, serving low density commercial, service, and light industrial uses.
Primary characteristics affecting capacity for commercial/service thoroughfares include:
- High volumes of automobile traffic;
- Low-density development requiring access from the thoroughfare; and
- The need for improved transit, bicycle and pedestrian facilities.
Balance capacity across modes. As travel demand increases over time, communities will need to address the seemingly constant need to increase capacity along major commercial thoroughfares. The effects of roadway widening can be devastating to these corridors, both economically and aesthetically, and the effort to increase capacity in this way may become increasingly irrelevant. Communities will inevitably decide to make improvements to the corridor that will not only make other modes more viable and more attractive but also to transform these sometimes blighted corridors into community assets.
The importance of vehicular capacity along these thoroughfares must be tempered by the varied needs of other modes, adjacent property owners and the surrounding community. Currently, these thoroughfares cater primarily to vehicles and create a hostile environment for bicyclists and pedestrians, who may also be transit users. Changing these circumstances will require an improved understanding of space allocation, as it relates to capacity. Safe, efficient, and attractive facilities for pedestrians, bicyclists, and transit users along these corridors would encourage use of these other modes. Modest tradeoffs in space allocation along the commercial/service thoroughfare can greatly improve capacity.
If we prioritize other modes of travel and offer people attractive travel choices other than their personal automobiles, then we are in effect reducing vehicular demand on the thoroughfare. Transit alone can significantly reduce single occupancy vehicular demand. To capitalize on this reduction in demand, though, we must prioritize transit, not simply accommodate it. Failure to prioritize other modes will minimize the capacity benefits.
Add capacity by providing attractive transit choices. There are a variety of measures available to help provide attractive transit choices along our commercial/service corridors, thereby reducing single-occupancy vehicle demand and improving capacity. Traditionally, buses use the general vehicular travel lanes, often requiring pull-outs at bus stops, which make it difficult for buses to reenter the stream of traffic and decrease the efficiency of transit use. Transit-only lanes can help prioritize transit as a mode choice along commercial/service thoroughfares with the highest ridership. 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. 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 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. 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.
Appropriate location of bus stops (mid-block, nearside, or farside). The most appropriate location typically depends on site-specific characteristics. The tables 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).
Support transit with pedestrian improvements. Transit prioritization is intrinsically linked to pedestrian prioritization, since virtually all transit users must at some point access the bus or train as a pedestrian.
Safe and efficient pedestrian accommodations must be provided to make transit 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.
Pedestrian improvements are important along commercial/service corridors, which are not served by transit. Whether it is reducing vehicle trips made by nearby residents, or by customers attempting to visit multiple commercial destinations in the corridor, pedestrian improvements such as safe, continuous sidewalks can have a big impact. Reducing these trips alone could have significant capacity benefits along commercial/service thoroughfares by reducing the vehicular volume along the street.
Consider multi-way boulevards where appropriate. Multi-way boulevards can be effective facilities in commercial/service corridors because they separate through-traffic from traffic desiring access to the abutting businesses. These thoroughfare types are less common than they once were in the United States, but they offer an effective solution to high-volume thoroughfares in commercial/service areas where access and mobility present significant conflict. There are a number of challenges facing the implementation of boulevards. They represent a large investment in infrastructure that is not always feasible, even when appropriate.
Additionally, their design requires intricate planning, with which many agencies, planners, and designers are unfamiliar. They can, however, be a very effective when there are equally compelling arguments that support the corridor as a place for commercial activity and as a through-way. See the PDF entitled Building a Boulevard for some introductory information on multi-way boulevards, and consult professionals who are familiar with them to assist in determining whether they are appropriate for a given commercial/service area.
Resources
PDFs:
- Florida DOT LOS Table
- 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:
- Last Updated on Tuesday, 24 March 2015