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.

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 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.  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 in Office Employment Areas

Primary characteristics influencing capacity for office employment thoroughfares include:

• Large facilities that generate high number of trips in peak hours;
• High potential for bicycle and transit mode choices; and
• "Sustainable Commuting"

Depending on the target speed established for office employment 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.

Credit: CH2M HILL

Credit: CH2M HILL

Office employment areas are typically comprised of large office buildings that house high numbers of employees, all of whom commute in some way to and from their place of employment.  One of the greatest challenges in these areas is the fact that the large majority of employees arrive and depart in generally the same time frame. This "rush hour" demand creates a variety of capacity issues along the thoroughfare and beyond, especially if most employees commute via single occupancy vehicles. In these location types, roadway planners and designers should avoid automatically increasing the number of lanes along the thoroughfare and at intersections to service the ever-growing projections for single-occupancy travel. 

Providing more lanes may be warranted in certain circumstances, but choosing to do so has several consequences. First, it requires more space in what is usually a very space-constrained environment. That space has to come from somewhere, and it usually means sacrificing important space in the pedestrian realm. Secondly, more lanes create wider thoroughfares. Wider thoroughfares are not always undesirable, but in general they create a less friendly environment for pedestrians. Lastly, the spike in peak hour travel in office employment areas is often significantly higher than travel throughout the rest of the day. This in many cases creates a thoroughfare that is oversized for the majority of the day.

Creating great thoroughfares will require a different kind of thinking about planning for future capacity in office employment areas. There are more efficient ways to meet the rush hour travel demand experienced in employment centers than continuing to accommodate single-occupancy vehicle (SOV) travel along the thoroughfare. As described in the general comments above, single occupancy vehicles place a much greater burden on thoroughfare capacity than high-occupancy vehicles, buses, or light rail. Implementing measures to prioritize other modes can reduce employee dependency on the SOV. Until these other modes become attractive commuting choices, they will not provide the capacity benefits they could.

Commuters that drive past transit vehicles struggling to merge back into traffic from a bus stop turnout will not see transit as a more attractive commuting choice.  If, however, they watch the bus pass by in a transit-only lane or queue jumper, they may think differently about their commuting options. Similarly, seeing a bicyclist driving in the gutter, dangerously close to high-speed traffic will not promote bicycling as a commuting option. If we really want to encourage other commuting options to improve capacity, then we must go beyond our typical minimum provisions. We must provide facilities that are as safe and efficient as commuting by personal vehicle.

Bicycle Commuting Options. Avid bicyclists are typically comfortable riding on almost any road. They assert their position within the travel way to command respect from vehicular traffic. In spite of their comfort level in such conditions, they are often too exposed to traffic traveling at high speeds. Collisions can result in serious injury or even death.

For the general population who may be less comfortable bicycling on the street, commuting in this manner is not safe enough to consider as a commuting option. An unstriped sliver of pavement at the edge of the travel way is simply too intimidating for most of us to consider this as a commuting choice. Couple these typical conditions with speeds often in excess of 40 mph, and it's no wonder why so few commuters choose to bicycle to work. But what if we provided safer conditions for bicyclists? What if we reduced vehicular travel speed on streets where bicyclists travel? If we were to proactively create such conditions, there is no doubt that bicycle commuting would increase. Below are several measures that should be considered as ways to promote bicycle commuting:

Transit Commuting Options. Transit is another great way to alleviate the vehicular congestion that often plagues office employment thoroughfares. A variety of treatments and options are available to help promote and prioritize transit as an attractive commuting choice.

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 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.

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 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.  

One of the important automobile provisions necessary to serve these businesses is parking. The Parking section has a more detailed discussion of specific office area applications. Parking is an important mixed-use element that has immediate impact on employment centers, but also an impact on street capacity. 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. This is a choice to be weighed carefully in every situation. 

Sustainable Commuting. In today's world of diminishing natural resources, we ought to reward those employers and employees that choose sustainable commuting options. Carpooling, transit, bicycling, and walking are all commuting choices that minimize resource consumption and environmental impact (air pollution, noise pollution, etc.). These choices also help to improve the capacity of thoroughfares in office employment areas. Government agencies and corporations should encourage their employees to participate in these sustainable commuting options via tax incentives, vouchers, rebates, or other means to further the goal of prioritizing these other modes of commuting. If we want to change our commuting habits, we must collectively embrace these sustainable commuting options.

• Maximize the outside travel lane width and provide clearly identifiable bike lane striping.
• Bike lanes can also work well along corridors with designated transit lanes, because transit lanes are often relatively low-volume.
• Direct trucks and other large vehicles to designated truck routes whenever possible to minimize conflicts with bicyclists.
• Keep motor vehicle travel speeds as low as practicable
• Ensure that drainage grates are clearly visible and bicycle friendly.
• Provide secure, visible bicycle racks to discourage theft, as in the image at right.
• Be vigilant about pavement maintenance and repair, especially in bike lanes; large potholes and cracks can be a serious hazard for bicyclists.

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:

• 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)