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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 Office/Employment Areas:

Employment corridors are usually comprised of numerous large office employment centers that generate a high travel demand during the morning and evening rush hours. Promoting multi-modal access throughout these corridors can help reduce peak period congestion. Providing a variety of travel options also creates a more attractive environment for prospective employees and businesses. Office/employment areas have:

  • Transit access
    Credit: CH2M HILL

    High travel demand during rush hour;

  • Significant transit presence; and
  • Significant pedestrian presence.

As the regional transit system in St. Louis continues to expand, provisions for bus and light rail commuters should be a centerpiece of street design in office employment areas. Providing adequate and efficient access for buses along the street will encourage transit use and limit the impact of bus service on vehicular roadway capacity. 

Plan for pedestrians and bicycles. 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. 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.

Bicycles are another important mode along these streets, and they will also be at conflict with access points along the corridor. Whether bicyclists 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 to the extent practicable. Bicycle-specific measures should be provided at intersections to prioritize bicycling at these locations. 

The crossroad access points/intersections will generally be either signalized or unsignalized. Signalized access points can provide a safer pedestrian environment by including pedestrian crossing signals and, if appropriate, restricted turning movements during pedestrian crossings. 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. 

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.

Reduce points of conflict. Access points create conflicts for through-moving vehicles, pedestrians and bicycles. Drivers are typically focused on vehicular traffic when attempting to enter or exit an arterial, and may fail to see a pedestrian crossing the driveway or a bicycle approaching.  Drivers fixated on traffic approaching from the left may edge out onto the arterial without looking again to the right to check for pedestrians. In areas with a significant pedestrian presence, the number of driveways should be minimized to the extent practicable. At higher volume access points, signage reminding drivers to watch for pedestrians can be used to minimize conflicts.

Provide access to/from parking garages on secondary streets. Office/employment areas often have significant parking requirements. These lots and garages may serve as park-n-ride transit lots or lots serving the commuting employees who work in the respective place. The parking capacity often provided by these facilities is significant, and places a concentrated demand on the surrounding road network during peak hours. 

Intersection diagram
Source: FHWA

The best way to manage the provision of access for these large parking facilities is to locate them 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.

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. See Chapter 10 of ITE's Context Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities for more information about queue jumpers and prioritized transit. 

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 crossings, perhaps even mid-block pedestrian signals, should be provided in close proximity to median transit 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 provide additional information on these subjects.

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. 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. 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.