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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 increases the number of potential conflicts along an arterial.

Intersection conflicts
Source: FHWA

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
  • 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
Signalized intersection
Credit: CH2M HILL

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 intersection
Credit: CH2M HILL

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. 

Access along commercial street
Credit: CH2M HILL

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.

Raised median
Credit: CH2M HILL

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.

Landscaped median
Credit: CH2M HILL

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.
  • Raised medians provide the opportunity for attractive landscaping.
    Low profile barrier
    Credit: CH2M HILL

    Landscaping, 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. 

Pedestrian refuge
Credit: CH2M HILL
Pedestrian refuge collage
Credit: CH2M HILL

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.