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Access

Article

Access 1
Source: FHWA

Access refers to the demand for vehicular entry and exit to and from driveways and crossroads that intersect with a thoroughfare. 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 thoroughfare. Each access point represents a potential conflict between turning and through-moving vehicles, pedestrians, and bicycles on the thoroughfare, as illustrated in the diagrams at right.

Access 2
Source: FHWA

Greater access point density increases the number of potential conflicts along a thoroughfare.

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 thoroughfare, while ensuring maximum safety for all users. Typically, a thoroughfare's functional classification guides the location and spacing of access points.

Access management plans must be site-specific and place-based.

Access 3
Source: FHWA

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 thoroughfare access point decision.

Balancing competing interests is critical to successful implementation, and is perhaps one of the biggest challenges in designing great streets.

Access 4
Source: FHWA

Allowing unlimited access points would undermine the safety and efficiency of the thoroughfare. 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 thoroughfares 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 thoroughfare 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 thoroughfare 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 thoroughfare safety
  • Improve traffic operations
  • Protect taxpayer investment in the thoroughfare
  • 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. A city, county or DOT is responsible for examining individual development studies in the context of the area. Developments should be planned with the region in mind. 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 thoroughfare seems reasonable, but the combined impact of several projects can create traffic problems along the thoroughfare.

Access along Downtown Main Streets:

Characteristics that influence access along Downtown Main Streets:

  • Numerous abutting smaller parcels
  • Lower travel speeds
  • Significant pedestrian presence
  • Transit access requirements

Access points in downtown areas generally refer to crossroads or intersections. Downtown areas are typically comprised of numerous small parcels, usually commercial or mixed-use in nature. These parcels rely primarily on foot traffic, rather than vehicular access onto the property. Patrons walk from nearby neighborhoods, transit stops, or a parked car or bicycle. While some driveways to individual parcels may exist, they are infrequent.

The crossroad access points 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 and minimum spacing is beneficial for the efficiency of the greater thoroughfare. If signals are spaced regularly, the progression of traffic along the thoroughfare is better served. Minimum signal spacing for downtown areas is typically one-quarter mile.

Signalized access points 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 major roadway itself. They may represent a minor road crossing, a parking lot entrance, or an atypical larger parcel entrance.

Curb cuts and driveways should be minimized.

U-turn and signage
Source: CH2M HILL

Cross access and a minimum access point density should be promoted along downtown main street corridors. In a downtown, the quality of the area is determined by the pedestrian and architectural environment, not the thoroughfare. Drivers attempting to enter the thoroughfare from a driveway are often fixated on traffic approaching from the left, and may edge out onto the road without looking back to the right to check for pedestrians. 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.

Raised medians can be effective in reducing the number of access-related conflicts along downtown main streets. Raised medians can be an effective measure to reduce the number of access-related conflicts along downtown main streets. Raised medians restrict left turn movements to only 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).

Landscaped median
Credit: CH2M HILL

Raised medians also provide the opportunity for attractive landscaping that can aesthetically enhance the thoroughfare (see the safety section of this guide for more information about issues related to trees or other fixed objects in the median).

When raised medians are used, it may be appropriate to allow u-turn movements at select locations to provide sufficient access to both sides of the thoroughfare, as in the image at right. Simple and highly visible signing can help drivers navigate these maneuvers.

Raised medians also provide opportunity for attractive landscaping to enhance the aesthetics of the street.

As plans are being developed for reconstruction of downtown thoroughfares, often the communi­ties involved seek to improve the livability and pedestrian environment. Many com­munities may want to improve the image of the corridor as a means to promote economic revitalization and land use development. Therefore, landscaping, including street trees, are desired elements of the streetscape. Most design guides for developing sidewalks along roadways suggest that a landscaped buffer be placed between the street and the sidewalk, and the preferred location for trees is in that buffer.

When raised medians are included as part of an access management plan, the community often wants trees in the median. Other vertical objects that may be desired can include signal and luminaire poles in the median or along the sidewalk. The wide cross-sections on major thoroughfares increase the need to place poles in the median (e.g., due to excessive signal mast arm lengths).

The concern that planners and designers face is that some believe these features to be new obstructions within the 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.

Another approach being tested in Washington State is an in service review to evaluate actual safety performance. The in service review establishes an agreement to monitor safety performance of the constructed features and to implement appropriate mitigation measures if necessary. Other 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.

Raised medians provide opportunity for pedestrian refuges. Downtown thoroughfares may be quite wide due to the number of lanes required, the median width, and the on-street parking requirements. The long crossing distances can be accommodated for most pedestrians. However, there is increased chance that some may be stranded in the middle of the street as the walk phase changes. Therefore, pedestrian refuges in medians are proposed as a feature to improve the pedestrian environment.

Traditional design reviewers are concerned that pedestrians should not be encouraged to remain in the median. Other concerns are that the minimum refuge width should be 6 feet wide. And, if a raised island extends beyond the crosswalk into the intersection, concerns have been raised that turns by large trucks and by u-turn vehicles are affected.

Some agencies plan signal operation for pedestrians to cross only half of the street in a phase. Therefore, under that circumstance, the refuge should be designed with a larger width and greater protection should be provided. When the intent is to have pedestrians cross the entire street, then a larger refuge width becomes counter-productive. With seven or more lanes, a percentage of pedestrians will not be able to cross the full width. Therefore, a pedestrian actuation button should be considered for the median.

Design of the refuge should include application of turn tem­plates for trucks and the design vehicle for u-turns. Ideally, the end of the median refuge should extend across the crosswalk. To address concerns that large vehicles may hit the refuge, some agencies are using a mountable nose with either a sloped face or reduced height. Other measures include reflectors and raised delineators and the use of colored concrete to make the refuge conspicuous. The figure below/at right shows the use of a pedestrian refuge island to address safety concerns. The image on the left is a collage of pedestrian refuge designs from cities throughout the U.S. and Europe.

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

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.

When raised medians are used in a downtown thoroughfare, u-turn movements should be permitted at select locations to provide sufficient access to both sides of the street. 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 downtown thoroughfares, 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, which are often prevalent in downtown corridors
  • 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 thoroughfare 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.

Often the spacing between signalized intersections is 1/3 mile or greater. Therefore, it is desirable to develop mid-block u-turn locations.

U-turn figure
Credit: CH2M HILL

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