Choices & Guidelines
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- Last Updated on Tuesday, 24 March 2015
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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 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.
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Public utilities are a frequently overlooked element of arterial street design, despite the significant implications their placement, maintenance, and design have on roadway functionality and cost. Both underground and overhead utilities occupy a significant space within the right-of-way. See the attached PDF document for a diagram of utilities.
Utilities which are commonly overhead:
- Telephone
- Cable television
- Overhead lighting
- Electricity
Utilities which are typically underground:
- Water
- Natural gas
- Irrigation (sprinkler systems)
- Sanitary sewer
- Storm sewer
The poles used to support overhead utilities can present a roadside safety risk in some corridors. Specifically, 10 percent of all fatal, fixed-object crashes are a result of motor vehicle collisions with utility poles supporting overhead utility lines. If utilities must be overhead, it is imperative that the poles and cabinets are located in the areas where they are least likely to be struck by an errant vehicle. AASHTO’s Roadside Design Guide offers several design considerations for maximizing safety when roadside overhead utilities are present.
- Locate power and telephone lines underground, whenever possible. Burying these lines can be costly, especially in retrofit situations but the improvement in safety, appearance, and mobility for pedestrians is worth it.
- Maximize the lateral distance between the vehicular travel way and utility poles. AASHTO recommends a minimum of 18 inches between the back of the curb and the roadway. The resulting tradeoff is that the utilities are then closer to building frontages.
- Reduce the number of utility poles along the street. Maximize the spacing between utility poles and whenever possible, combine multiple utilities on a single pole (e.g. combine overhead lighting with a traffic signal and perhaps even a power line).
- Use a breakaway pole design. This will minimize the impact and severity of collisions.
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Credit: CH2M HILLUse traffic barriers to shield poles. While barrier curbs lose effectiveness at speeds over 30 mph, low-profile barriers (as shown at right) can be quite effective for higher speed arterials (up to 45 mph).
- Preserve pedestrian walkway. Overhead utilities significantly affect the character of the streetscape. In addition to being unsightly, above ground utility poles are often located along or even in the middle of the sidewalk, encroaching on the pedestrian walkway. The clear pedestrian walkway should be a minimum of five feet wide, even when poles are present. In downtown areas sidewalks should be wider to accommodate high volumes of pedestrian traffic and pedestrians using mobility aids.
- Consider maintenance of utilities in the planning process. Utility maintenance (overhead or underground) should be considered when locating utilities and other roadside elements such as trees, street furniture, traffic signs, and drainage inlets that could potentially impede or prevent access.
- Coordinate early and often. Utility coordination is an essential component of the planning and design of great streets. Most streets have a number of utilities, each of which may be owned and managed by a different agency. Frequent and early coordination with these agencies can save time and money, especially when service lines are being upgraded or relocated. Coordination between roadway planners and utility agencies can improve design and lower the costs associated with roadway construction or improvements. The agency responsible should understand utility plans for the corridor and provide the appropriate utility companies with street design plans early in the process to solicit comments. Close coordination can also minimize impacts associated with construction, particularly for adjacent property owners.
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Credit: CH2M HILLLocate utilities before making improvements to an existing roadway. A thorough subsurface utility investigation should be performed before beginning work on retrofit projects to avoid discovering unexpected utility lines, and the associated costs and setbacks.
Existing plans or “as builts” can be used in combination with detection technologies such as ground penetrating radar to identify unrecorded underground utility lines along a corridor. A relatively small investment of time and money early in the planning stage can help minimize unanticipated costs during construction.
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Improve the aesthetics of utilities through design, signage, lighting, and seasonal decorations. There are a variety of simple, innovative ways to transform utility hardware into attractive streetscape enhancements. For example, ordinary traffic signal cabinets can be decorated with colorful, artistic murals linking the street to the surrounding land uses, as shown in the images below. Other improvements to utility infrastructure can involve partnerships with local businesses and residents to provide and maintain amenities, such as flowers and plants, lights and decorations. Involvement by citizens and local businesses can reduce costs, enhance community buy-in, and build mutual interest in creating and maintaining great streets.
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Although most transportation research, published articles, and popular discussion focus on the movement of motor vehicles through roads and intersections, cars actually sit in parking spaces 95 percent of the time. In many urbanized areas, parking assumes 20-40 percent of the land surface area.
The design and location of parking is one of the most important elements of great streets. The quantity, location, management, cost, and design of parking depend heavily on the adjacent land use the parking is serving. For example, parking design in downtown areas is very different than parking at a suburban office complex. Still, there are several parking principles that permeate all land uses.
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Ensure on-street parking: On-street parking is the most valuable type of parking for several reasons:
- It creates a physical and psychological buffer between pedestrians on the sidewalk and moving traffic.
- It presents the best access to the front door of retail, residential, and commercial destinations.
- It limits the need for off-street parking facilities. Off-site parking facilities use valuable land, require additional curb cuts through the pedestrian realm for access, and present challenges to creating good urban design. Additionally, in urban areas, off-street parking facilities can be extremely expensive.
- On a per space basis, on-street parking takes up less land area than other forms of parking because the ramps, driveways, and aisles needed in parking lots and structures are absorbed by travel lanes themselves.
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Place parking behind buildings: Fronting streets with buildings (as opposed to parking) creates a more interesting, pedestrian-friendly environment. Locating parking behind buildings also allows driveways/access points to be placed on lower volume side streets, presumably with fewer pedestrians. Placing driveways (including those to parking lots) on side streets creates a more continuous pedestrian frontage, reduces the potential for pedestrian-motor vehicle conflicts, and eliminates mid-block left hand turning movements on the higher volume street - a leading cause of mid-block congestion.
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Encourage shared parking: Peak parking demand for adjacent land uses often occurs at different times of the day. For example, a bank and a neighboring movie theater could feasibly share spaces as their parking demand peaks at different times. Although such arrangements are more common in mixed used districts, shared parking can be implemented in all place types. Shared parking decreases the need for off-street parking spaces and lots, which imparts many of the benefits mentioned above. See the Urban Land Institute’s Shared Parking (Smith. Shared Parking, Second Edition. ULI and the International Council of Shopping Centers. 2005) for more information about shared parking.
Source: ADAAG -
Provide handicapped parking: Federal law requires that handicapped parking be provided in all designated parking areas. The Americans with Disabilities Act (ADA) sets guidelines for handicapped parking requirements for all new construction. The minimum number of required handicapped spaces is based on the total number of spaces in the parking facility, as outlined in the figure at right.
The location of these spaces is also regulated, as explained below in the ADA Accessibility Guidelines:
“Accessible parking spaces serving a particular building shall be located on the shortest route of travel from adjacent parking to an accessible entrance” (ADAAG 4.6.2).
Although there are established national standards, individual municipalities often set their own standards for handicapped parking. A 1990 study of retail shopping centers found that the number of handicap spaces in commercial areas varied between 0.7% and 2.0% of the total parking supply (Weant and Levineson. Parking Spaces. McGraw Hill. 1999).
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Include bicycle parking: Although automobiles typically come to mind when parking is mentioned, great streets have provisions for all modes, including adequate and secure bicycle parking. There are no national standards for bicycle parking, and local requirements vary widely. One of the most convenient methods for establishing appropriate bike parking design is to view the ordinances of municipalities with high bike mode shares. The city of Madison, Wisconsin, for example, provides excellent guidelines on the appropriate quantity, location, and design of bike parking. The city of Portland, Oregon also has an excellent web page with guidelines for installing bicycle parking.
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Minimize supply: Parking is often oversupplied, which creates a number of design challenges. A 2003 study of 42 parking lots during the holiday season found that the average occupancy was less than half (Gould. “Parking: When Less is More.” Transportation Planning, Vol.28, No.1. Transportation Planning Division, APA. Winter 2003).
Source: City of SeattleRetail shopping centers with massive parking facilities that are rarely (if ever) full are a common sight in most communities. Parking is oversupplied because the minimum parking requirements for residential and commercial development is often set at the annual maximum expected demand, leaving excess parking for much of the year.
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Ensure delivery parking: Delivery parking must be provided in most, if not all, place types, but is particularly important in areas with a high concentration of retail shops and restaurants. Alleys are ideal locations for temporary truck parking, allowing back door delivery access away from customer parking and entrances. When alley use is not feasible, special loading zones can be designated. The image at right is an example of signage used in a loading/unloading zone.
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Source: FHWA An intersection is defined as the area where two or more roadways join or cross, but also includes elements of the functional area, such as intersection approaches, medians, sidewalks, bike lanes, and other roadside features.
The image at right highlights the physical and functional areas of an intersection. Intersections on great streets must serve all modes of travel.
Automobiles, transit vehicles, pedestrians, and bicyclists should all be given adequate time, space, and directional cues to safely proceed through intersections and continue traveling along the arterial. Balancing the needs of all users at multimodal intersections, while maximizing substantive safety is a complex and important challenge.
- Intersections are points of conflict where modes of travel converge, as illustrated in the image at right.
- Intersections should be carefully designed to include and prioritize the most appropriate place-specific design elements.
- Intersecting roadways should cross at an angle of at least 75 degrees, ideally 90 degrees. When the angle of intersection is less than 60 degrees special design treatments may be needed to ensure a reasonable level of safety.
- At intersections, medians can be used to provide separation between opposing traffic, channelization for turn lanes, and refuge for pedestrians.
- Medians with landscaping and tree plantings can also be used to improve intersection (or roadway) aesthetics, although care should be taken not to affect driver or pedestrian visibility and sight distance.
Movement through intersections is controlled using yield signs, stop signs, roundabouts, and traffic signals. The appropriate type of control for a given intersection depends on the place type and the amount of pedestrian and vehicular traffic.
The MUTCD provides guidance for selecting the appropriate type of control for various intersection conditions (see the following links for general information and specifics about signal warrants).
Different traffic control devices impose varying degrees of delay on pedestrians and vehicles passing through the intersection. The overall efficiency and capacity of a roadway is limited by the delay experienced at its intersections.
Some agencies and municipalities continue widening intersections by adding exclusive, dual, or even triple turn lanes in an effort to minimize delay along the arterial. While these improvements do increase an intersection's vehicular capacity, they also render the intersection more difficult for other modes of travel (especially pedestrians) to navigate. Because turn lane additions are typically retrofit projects they can significantly impact surrounding residences, businesses, and land parcels.
Designing intersections for great streets requires balancing competing needs, interests, and values, and responding to the unique circumstances of each street. Planners, designers, policy makers, and local stakeholders should collaborate to develop a community vision which can be used to guide the design and construction of intersections and roadway improvements.
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Source: FHWA Access refers to the demand for vehicular entry and exit to and from driveways and crossroads that intersect with an arterial. These driveways and crossroads are commonly referred to as access points. Access points present a number of planning and design challenges and potential hazards along the roadway. Each access point represents a potential conflict between turning and through-moving vehicles, pedestrians, and bicycles on the arterial, as illustrated in the diagrams at right. Greater access point density increases the number of potential conflicts along an arterial.
Access management refers to the regulation of access point location and spacing and is a crucial part of creating a great street that is safe for all modes. Access management is intended to balance mobility for through-traffic and access for vehicles attempting to enter or leave the roadway, while ensuring maximum safety for all users. Typically, a roadway's functional classification guides the location and spacing of access points.
Access management plans must be site-specific and place-based. The figure at right depicts the traditional relationship between access and functional classification.
At the top of the functional class hierarchy (principal arterial freeways), mobility is provided at the expense of access; at the bottom (the local road system), extensive access is provided, which limits mobility. Planning and designing great streets requires finding the most appropriate balance between access and mobility, according to place type. While local guidelines can sometimes be useful, standard solutions based solely on functional classification rarely produce desirable outcomes.
Tradeoffs are inherent in every roadway access point decision. Balancing competing interests is critical to successful implementation, and is perhaps one of the biggest challenges in designing great streets. Allowing unlimited access points would undermine the safety and efficiency of the arterial street. Conversely, prohibiting all access would render adjacent properties essentially worthless.
Access must be considered on a case-by-case basis. The owning transportation agency (the state, county, or local municipality), controls access rights along roadways within a jurisdiction. Most agencies have policies in place to regulate new and existing access point development. Nonetheless, access plays such a critical role in determining the environment along a roadway that planners, designers, and stakeholders are encouraged to carefully examine projects on a case-by-case basis.
Site-specific conditions and community objectives should always be taken into account when deciding how to manage and control roadway access. Specific spacing requirements will vary based on site-specific conditions. The ITE Traffic Handbook Table 10-5 provides guidance for minimum spacing requirements.
MoDOT's Access Management Guide identifies four major goals of access management:
- Improve roadway safety
- Improve traffic operations
- Protect taxpayer investment in the roadway
- Create better conditions for non-motorized modes of travel
MoDOT emphasizes that the guidelines are intended to allow for flexibility when necessary, and their overarching goal is to provide a safe and efficient transportation system while balancing the need for access to abutting land uses. Broad standards should not be applied without careful consideration of a project's unique characteristics.
Traffic impact studies should consider the large-scale transportation network (current and planned) before access permits are granted. Developers are typically required to conduct a traffic impact study before gaining approval for new developments. Unfortunately, each individual development usually conducts its own study, failing to capture the cumulative impacts of all proposed developments in the area. In such cases, a project may be approved because its impact on the roadway seems reasonable, but the combined impact of several projects can create traffic problems along the arterial.
Access along Commercial/Service Thoroughfares:
Access points in commercial areas refer to crossroads, intersections, and driveways. Commercial and service corridors typically are less dense than more urban commercial areas. These thoroughfares carry significant volumes of traffic and require vehicular access for business operations, including customers. They include "big box" stores, commercial strip development with restaurant and hotel chains but also smaller, locally-owned businesses that rely on vehicular access, such as service stations, and other light industrial uses. While these corridors are unlikely candidates to become "pedestrian places," there are important improvements, related to access, that can enhance safety for all modes and allow travelers more options for mode of travel.
Key access considerations for commercial/service corridors:
- Improving the safety for all modes
- Balancing the competing needs for multimodal access, parking and through-traffic
Design intersections for pedestrians and bicycles. Improving pedestrian accommodations and taming traffic along the street can promote foot travel between businesses. Such improvements also make transit a more viable option. In commercial areas, crossroad access points will generally be of two types: signalized or unsignalized. Both types have important characteristics to consider in the development of great commercial streets. Regular spacing of signalized intersections (no less than ¼ mile apart) promotes a more efficient flow of traffic on the arterial.
Signalized access points typically provide safer crossing opportunities for pedestrians through the provision of pedestrian crossing signals and if appropriate, restricted vehicle turning movements during pedestrian crossings.
Unsignalized intersections occur more frequently along the roadway and create more potential conflicts between cars, pedestrians and bicycles. These intersections are usually controlled by stop signs on the minor road without stopping traffic on the major arterial. Because not every intersection can be signalized, it is important to make improvements to unsignalized intersections such as marked pedestrian crossing with signage, where appropriate and stop bars that do not encroach on pedestrian crossings.
Provide pedestrian connections between destinations. When buildings are set back from sidewalks by large parking lots, pedestrians must have safe connections from the public sidewalk to the storefront AND between storefronts. These connections should provide safe crossings and should be appropriately protected from traffic. Ideally, parking would be located behind the commercial business so that any setback is minimized and pedestrians may enter businesses directly from the public sidewalk.
Consolidate access. Many existing arterials have numerous driveways and curb cuts and in some cases, unrestricted access to the roadway, where the entire property frontage functions as a driveway. These conditions are unsafe for all modes, but they are particularly threatening to pedestrians and bicyclists, who are already the most vulnerable. Bicyclists and pedestrians are unable to reasonably predict traffic movement and are under constant threat of turning vehicles. For properties with continuous access to the roadway, consolidate access to a property as a single driveway.
For corridors with numerous driveways, consolidate access by using shared driveways. By consolidating access to private property in this way, it is possible to reduce the conflicts of turning vehicles with pedestrians, bicycles and through-moving vehicles. When driveway consolidation results in higher volumes (more than 50 vehicles per hour), it may be necessary to provide auxiliary turn lanes.
Provide access from secondary roadways. Commercial/service thoroughfares have significant access and parking requirements. The best way to provide access to these businesses may be to locate the access points off of the arterial street, either on cross roads or "backage" roads. Such provision of access will minimize conflicts on the arterial resulting from direct driveway access.
When this is not possible and direct driveway access onto the arterial is required, spacing of the driveway(s) with respect to other crossroads and driveways is very important. These driveways at a minimum should be located outside of the functional areas of nearby intersections. Preferably they should be spaced as far from the next closest up- and downstream access points as practicable.
Use raised medians, when appropriate. Ideally, thoroughfares in office/employment areas will have vehicle speeds that make for a safe pedestrian environments and narrow crossings at intersections. However, it is possible that a thoroughfare through office/employment areas could be improved with the use of raised medians, when there is adequate right-of-way and need for the following improvements.
- Raised medians reduce the number of access-related conflicts. Access to driveways is confined to right-in, right-out movements, reducing the number of potential conflicts from nine (9) to three (3) at a standard 3-legged intersection (see Oregon PDF for additional detail on right-in, right-out channelization design). Left turn movements are restricted to access points across from a gap in the median. Because turning movements are restricted, the network must allow appropriate access to both sides of the street through allowed u-turns, roundabouts, and/or an improved (in some cases, one-way) grid on the neighboring streets.
- Raised medians can help reduce vehicle speeds by creating more visual friction in the thoroughfare. The visual impact of a median can slow vehicle speeds and appropriately calm traffic to improve safety along the thoroughfare.
- Raised medians provide opportunity for pedestrian refuges, when crossing distances are long. Some commercial/service thoroughfares in the region have too many travel lanes to ensure safe pedestrian crossings within a single light cycle. Medians can help alleviate this problem. Raised medians with this purpose should include a preferred minimum refuge width of 6 feet if pedestrians are intended to remain in the refuge during a signal cycle. These medians should extend beyond the crosswalk towards the intersection for pedestrian safety. Design of the refuge should include application of turn templates for trucks and the design vehicle for u-turns. Reflectors and raised delineators as well as the use of colored concrete can increase the visibility of the median refuge as well as the pedestrians using it. The figure below/at right shows a pedestrian refuge island that addresses some of these safety concerns. The image on the left is a collage of pedestrian refuge designs from cities throughout the U.S. and Europe. Although wider refuges are desirable, narrow width (e.g., 4-foot-wide) pedestrian refuges do meet minimum width requirements identified in many of the U.S. documents on urban thoroughfare design. AASHTO's "Policy on Geometric Design of Highways and Streets" (2001) says that 4 feet is the minimum width. ITE's Design and Safety of Pedestrian Facilities (March 1998) states that the minimum width for pedestrian refuge island is 4 feet.
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Raised medians provide the opportunity for attractive landscaping.
Credit: CH2M HILLLandscaping, including street trees, can be an important component of this streetscape enhancement. Most design guides suggest that a landscaped buffer be placed between the street and the sidewalk, preferably with street trees. Trees can also be placed in raised medians, assuming they do not impede visibility. See the safety section of this guide for more information about issues related to trees or other fixed objects in the median. Landscaping, along with other objects in the median should allow for the recommended clear zone. AASHTO recognizes that "space for clear zones is generally restricted" and suggests that a "minimum offset distance of 18 inches should be provided beyond the face of the curb." AASHTO also suggests that "since most curbs do not have a significant capability to redirect vehicles, a minimum clear zone distance commensurate with prevailing traffic volumes and vehicle speeds should be provided where practical." Design speed is an important factor relating to crash risk and severity. Other considerations include review of accident history involving lateral obstructions on the project of concern or for similar thoroughfares.
Washington State is currently conducting an in-service review to evaluate actual safety performance related to obstructions in medians. The in-service review establishes an agreement to monitor safety performance of the constructed features and to implement appropriate mitigation measures if necessary. Some design mitigation concepts include use of shoulders and auxiliary lanes to increase clear zone separations. For example, curb lanes used for transit and turning vehicles only, would have lower volumes and lower speeds than through lanes. Therefore, these auxiliary lanes provide a separation between the through lanes and adjacent vertical objects. Raised curbs, raised planters and barriers are also being evaluated as means to redirect vehicles or reduce severity of crashes. Lighting in the median may be considered to improve visibility of medians for drivers under night conditions. The figure here shows a low profile barrier that has passed crash testing and is being used by CH2M HILL on a median for a commercial/service corridor.
The image below, at left is an example of a pedestrian refuge that addresses safety concerns. The image at right is a collage of pedestrian refuge designs from cities throughout the U.S. and Europe.
Try to avoid Two-Way Left-Turn Lanes (TWLTLs) where transit, bicycle, and pedestrian activity is planned. The St. Louis region is no stranger to TWLTL's. For commercial thoroughfares, TWLTL's present a number of dangers for vehicles, as well as pedestrians and bicycles. TWLTLs reduce the ability to predict turning movements and as a result, create more conflict points for other modes as well as other vehicles.
They may be effective for providing access along arterials when:
- Traffic levels are moderate (roughly 10,000 to 28,000 vehicles per day);
- When turning movement demand is high; and
- When the density of commercial driveways is relatively low.
Driveway consolidation and appropriate spacing is vitally important, when TWLTLs are used. As traffic volumes increase (in the realm of 24.000 to 28,000 vehicles per day, depending on site-specific conditions) most TWLTL's begin to lose their effectiveness. At these traffic levels, it becomes more difficult for drivers to adequately select safe gaps in the traffic streams through which they want to turn. Driveway density is also a contributing factor to degradation of operation along TWLTLs; every driveway represents a conflict point, so more driveways means more conflicts for drivers to navigate.
For additional research and statistical data about the tradeoffs associated TWLTLs, please consult the resources associated with this article. One excellent body of research is provided by the Iowa State Center for Transportation Research and Education (Iowa State CTRE TWLT Data is just one of many resources offered). Beyond that, TWLTL's are relatively less safe than raised medians, and they are also less pedestrian friendly. TWLTL's create more conflict points for vehicles and pedestrians along the street.