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Access

Article

Intersection conflicts 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

Intersection conflicts
Source: FHWA

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.

Mobility vs. access figure
Source: FHWA

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.

Arterial vs. local vs. collector
Source: FHWA

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; and
  • 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 for Neighborhood Shops:

Characteristics that influence access in neighborhood shop place types:

  • Accentuated access-mobility conflict;
  • Significant pedestrian presence; and
  • Numerous smaller parcels abutting the roadway.

Neighborhood shop place types are different from downtown place types in that they are locally-serving and are not likely to be the primary business district for a city or town. Neighborhood shops may be located on the periphery of a community and/or along a higher-volume thoroughfare. The shops along these thoroughfares primarily serve the nearby residential neighborhoods with a variety of smaller businesses, grocery stores, banks, and restaurants. Clayton Road between Forest Park and Big Bend Blvd. is an example of a neighborhood shop place type. Because these place types are often on the periphery, it is not uncommon for the thoroughfares to also serve higher volumes of vehicular traffic. As such, the conflict between access to and from local shops and mobility along the thoroughfare is usually accentuated. This conflict has an immediate impact on access in neighborhood shop place types.   

Consolidate access points. Pedestrian presence along neighborhood shop thoroughfares is significant. Access points create conflicts between through-moving vehicles and pedestrians. Drivers attempting to enter the vehicular travel way from a driveway are often fixated on traffic approaching from the left, and may edge out onto the arterial without looking back to the right to check for pedestrians. In areas with a significant pedestrian presence, the number of driveways should be minimized to the extent practical. At higher volume access points, signage reminding drivers to watch for pedestrians can be used to minimize conflicts.

Another residual effect of the access-mobility conflict in these place types is related to vehicles desiring to turn from the street into one of the abutting parcels or crossroads. Deceleration lanes allow these vehicles to pull out of the through-travel stream before turning, resulting in safer traffic operations on the arterial. This treatment may necessitate acquiring additional right-of-way, and the potential impacts to adjacent parcels should be considered.  

Provide access from side streets, whenever possible. The need for direct driveway access points along thoroughfares can be minimized by providing access along lower speed crossroads, back roads, or alleys. Shared access, or the consolidation of access for multiple businesses, can improve the efficiency and safety of neighborhood shops corridors. It is important to remember that the number of driveways along an arterial street is proportional to the crash rate. Shared access drives should be monitored, though, to ensure that the access point is not being used excessively and causing congestion on the arterial. 

The crossroad access points/intersections will generally be of two types: signalized or unsignalized. Both types have important characteristics to consider in the development of great streets. For signalized intersections, regular spacing is beneficial for the efficiency of the greater arterial. If signals are spaced at regular intervals, the progression of traffic along the arterial is better served. Minimum signal spacing for these place types should be one-quarter mile.

Use signals at key intersections to facilitate pedestrian crossing. Signalized access points can provide a safer pedestrian environment by including pedestrian crossing signals and, if appropriate, restricted turning movements during pedestrian crossings. Unsignalized intersections are more frequent and create more potential conflicts between cars, pedestrians and bicycles. They are usually controlled by stop signs on the minor road without stopping traffic on the arterial itself. They may represent a minor road crossing, a parking lot entrance, or an atypical larger parcel entrance. Pedestrian visibility is always important, but even more so at unsignalized intersections.

Use raised medians, when appropriate. Ideally, thoroughfares in neighborhood shop areas will have slow vehicle speeds and narrow crossings to facilitate pedestrian ease and safety. However, it is possible that a thoroughfare through a neighborhood shop area could be improved with the use of raised medians.

Raised medians reduce the number of access-related conflicts by restricting left turn movements to the access points across from an opening in the median. 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).

Medians can help create a more urban-style access management plan but they also create issues for drivers seeking access to both sides of the thoroughfare. U-turns are generally not recommended in areas with a significant presence of pedestrians and bicycles because they reduce predictability. Careful network planning is important to accommodate the desired travel patterns and facilitate access to both sides of a thoroughfare.

  • Raised medians can help reduce vehicle speeds by creating more visual friction in the thoroughfare.
  • Raised medians provide opportunity for pedestrian refuges, when crossing distances are long. Raised medians with this purpose must be carefully designed, including 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.
  • Raised medians provide the opportunity for attractive landscaping (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 downtown thoroughfare project.

When raised medians are chosen, consideration should be given to the provision of u-turn movements at select locations to allow sufficient access to both sides of the thoroughfare.  Simple and highly visible signing can greatly assist drivers in identifying and navigating these maneuvers.  Because of the types of commercial and retail land uses along most neighborhood shop arterials, substantial property access volumes are of concern. Often, accom­modating U-turns is a partial mitiga­tion for changed property access due to the addition of a median. Design issues related to developing U-turn accommodations included: Lack of driver awareness or experience with U-turn maneuvers.

  • Selection of the appropriate design vehicle versus the limited available width of roadway and right-of-way.
  • The existence of right-turn/left-turn signal overlap operations.
  • The proximity of far-side bus stops to intersections.

The U-turn design should be developed to accommodate passenger vehicles with a design diameter (i.e., out-to-out distance) established as the minimum design standard.  When the street has only two lanes in each direction, the intersection corners can be modified with a taper to fit the U-turn diameter, or in some cases additional width is available from development of far-side bus stops. Signal operations should be modified to remove the right-turn/left-turn signal overlap. Special signs should be added to alert drivers to yield to U-turn vehicles. <insert uturnfigure.bmp, source = CH2M HILL> Often the spacing between signalized intersection is 1/3 mile or greater. Therefore, it is desirable to develop mid-block U-turn locations. Often there are constraints to spacing of left-turn pockets. It can be difficult to meet minimum storage length guidelines and taper length standards (e.g., 1 foot per mile-per-hour rates).

Also, concerns regarding safety for unprotected U-turns is another issue. When conducting a comprehensive design analysis, the following concerns should be weighed against the benefits of corridor-wide access management treatments and other mitigating factors:

  • Mid-block U-turns can help mitigate the affect of changed access by offering more frequent left/U-turn opportunities;
  • The mid-block U-turn location can help remove U-turn traffic from major signalized intersections-improving signal operations;
  • Generally the mid-block volumes (versus gaps in opposing traffic) do not require more queue space than one or two vehicles.

Consider multi-way boulevards where appropriate. Multi-way boulevards can be effective facilities in neighborhood shop areas because they separate through-traffic from traffic desiring access to the abutting shops. 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 neighborhood shop areas where access and mobility present significant conflict. One of the greatest challenges regarding these thoroughfare types is that they are a lost "art form", so agencies, planners, and designers are less familiar with their implementation. See the PDF entitled "Building a Boulevard" for some introductory information on multi-way boulevards, and consult a professional who is familiar with them to assist in determining whether they are appropriate for a given neighborhood shop area. <insert multiway_xsect.bmp, source = CNU > Lower speeds in these place types are essential for a variety of reasons such as improving a driver’s ability to perceive and react to conflicts created by access points. See Design Speed for more information. Although specific spacing requirements will vary based on speed, local codes and conditions, Table 10-5 in the ITE Traffic Handbook is a good place to start for minimum spacing guidance. Like most guidelines, driveway spacing is a choice. Standards and guidelines offer spacing values to consider, but should not be rigidly adhered to without considering the multi-modal implications created along the thoroughfare.