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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; 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 in Civic and Educational Corridors

Characteristics that influence access along civic and educational thoroughfares:

  • "Rush hours" generate significant multi-modal trips that often conflict with vehicular traffic attempting to go through the corridor;
  • Significant pedestrian and bicycle presence; and
  • Significant transit presence.

The rush hour characteristics of these place types present a variety of challenging elements. Students and employees generally arrive and leave during a concentrated time period in the morning and evening (or at the close of school), with a lesser concentration of travel occurring during the lunch hour as employees/students travel to restaurants or to run errands. In between these three peak periods, travel demand for all modes is usually less. The magnitude of the demand drop-off is dependent on the other types of land uses in the area.

Consider access control measures to encourage pedestrian, bicycle and transit activity. The rush hour peaking characteristics described above require planners and designers to have a clear understanding of the travel patterns of all modes during those time periods, both for existing conditions and planned future development. At intersections with high volumes, the following rush hour access control measures should be considered in an effort to prioritize pedestrian, bicycle, and bus travel:

  • Prohibition of left turns can improve traffic flow and improve pedestrian crossing conditions. If left turn prohibition is not an option, restricting left turns to a "protected only" phase (the phase when opposing traffic is stopped as vehicles are given the green left turn arrow) will eliminate the increased crash risk associated with "permitted" left turns (allowing traffic to turn left when they find what they believe to be an acceptable gap in the opposing traffic stream).
  • During the peak hours, certain turn movements may experience very high traffic volumes.  Special signal timing during peak conditions to prioritize these directional peaks should be used to optimize overall intersection operations. These signal timings should also consider peak hour demand for pedestrians.

Plan for pedestrian and bicycle movement. These thoroughfares should prioritize pedestrians along the street, and especially at the intersections. Intersections will provide the vast majority of vehicular access along the street, so it is vitally important to provide safe, efficient crossings and refuges for pedestrians at these locations. See the Intersections section of this guide for more detail.

The pedestrian environment along these corridors must be prioritized. Special signage to remind drivers to watch for pedestrians, as shown in the image at right, can be useful in managing this conflict when curb cuts and driveways are required.

Reduce points of conflict. Bicycles will also be at conflict with access points along the corridor. Whether cyclists travel along a marked bicycle lane, a wide outside lane, or along a shoulder, they will inevitably have to cross driveways and intersections. Where regular bicycle use is expected/desired, driveways should be limited. Drivers attempting to enter the thoroughfare from a driveway are often fixated on traffic approaching from the left, and may edge out onto the thoroughfare without looking back to the right to check for pedestrians. Bicycle-specific measures should be provided at intersections to prioritize bicycling at these locations. 

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 thoroughfare. Minimum signal spacing for these place types should be one-quarter mile. 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 thoroughfare 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.

Prioritize transit access. In these place types, transit must be prioritized. Mobility for transit, in the form of bus-only lanes and queue jumpers (also called queue bypass lanes) can help in this effort but require careful consideration of access along the street. In the figure at right, the no-build condition represents a typical transit provision where buses must share the outside travel lane. The queue jumper condition expands the no-build condition to include a bypass lane for buses at intersections.

The transit lane (could also be a shared transit/HOV/bike lane) condition provides a continuous bus-only lane throughout the entire corridor. These concepts can be very effective in prioritizing mobility for other modes, but they do have impacts on abutting access requirements.  When such measures are provided on the outside of the travel way, they will conflict with every driveway along the corridor.

Managing the location and spacing of such driveways, then, is imperative in order to encourage transit along the street. Failure to do so will inhibit the intended prioritization of transit. Spacing guidance should be developed on a case-specific basis, and it should be coordinated with local businesses and residents whose parcels abut the street. When transit lanes are provided down the center of the street, as in the image at right, conflicts with adjacent driveways are lessened. The center location, however, creates another set of conflicts to manage the movement of pedestrians to and from median transit stops. Pedestrian crossing, perhaps even mid-block pedestrian signals, should be provided in close proximity to median transit stops.

Consider access to bus stops. Such access must be provided efficiently and safely, without degrading operations for other modes along the corridor. Three general options are available for bus stop location: mid-block, near side, and far side. The tables at right describe the advantages and disadvantages to consider for each of these conditions. Each specific site condition will have characteristics that influence the best choice for bus stop location, and it is imperative that they be examined as such. There is no "one size fits all" solution that should be applied blindly across an entire corridor.

Regardless of location, clear and visible signing should be present to aid pedestrians in locating nearby bus stops. Chapters 9 and 10 of the ITE guide Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities provides additional information on these subjects.

Use raised medians, when appropriate. Ideally, thoroughfares in civic and educational areas will have slow vehicle speeds and narrow crossings to facilitate pedestrian ease and safety. However, it is possible that a thoroughfare through a civic or educational 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.