III. RESEARCH PROBLEM STATEMENT
Access management treatments inherently reduce conflict points along urban arterial roadways. The reduction in conflict has translated to a reduction in crashes that has been documented in numerous studies looking at before and after conditions when access management treatments (i.e., raised median installation, driveway consolidation) are implemented. Crash rate reductions as high as 50 percent have been reported in these analyses.
There is a far more limited understanding of the operational impacts of access management (particularly treatments in combination). Although improvements in travel speed and traffic flow have been documented in the literature, there is a need to better understand the circumstances under which access management treatments have a positive operational impact. Arterial roadway characteristics such as turning and through volumes, unsignalized access density, signalized access density, median type, turn-bay presence, and the combinations thereof, certainly affect corridor operations. A better understanding is needed of how these operational impacts change, as geometric characteristics and operational characteristics (volumes) change. Specifically, there is a need to understand how improved access management relates to average travel speed, travel time reliability, and preserved highway capacity.
IV. LITERATURE SEARCH SUMMARY
Some studies have been performed that address specific geometric characteristics and operational changes (i.e., different median treatments over a range of volumes). Several of these studies are in the form of unpublished, internal technical memorandums. Rarely do these studies incorporate a full operational analysis of a package of access management treatments (e.g., different median treatments and access density for a range of through and turning traffic movements). There is also a question of the transferability of the studies performed. Finally, the Highway Capacity Manual does not currently systematically and explicitly consider access management treatments in capacity analyses.
The need for this research was previously identified in 2005 as Problem Statement 36: Urban Streets Operational and Capacity Effects of Unsignalized Access on Urban Streets in Transportation Research Circular E-C081: A Research Program for Improvement of the Highway Capacity Manual as follows:
Although a great deal is known about signalized intersections and their effects on through traffic, much less is known about the effects of access points on the operation of urban streets between signalized intersections. The impacts on through-traffic speed can be profound when there is a high frequency of closely spaced access points (i.e., driveways and unsignalized public street intersections). These effects, of course, become more pronounced when there are substantial volumes in and out of many of the access points.
A limited number of case studies have documented degradation in travel speed and reductions in capacity for very specific driveway and roadway configurations. Currently, however, there is no procedure that can effectively predict the impact of a wide variety of driveway and roadway configurations on the average travel speed of through traffic.
The presence of access points along a roadway is assumed to have a frictional impact on through traffic. That is, average travel speeds will be marginally lower on roadways where access points are present, even if there is no ingress and egress activity at the points. The reduction in speeds becomes more pronounced as the level of ingress and egress activity at the access points increases. Simply stated, increased activity impedes more of the through traffic, forcing it to slow down and/or change lanes. The extent to which through traffic is impeded is a function of (1) the movements allowed in and out of the driveway, (2) the volume of these movements and the through traffic, (3) the design of the driveway, and (4) the cross section of the roadway. While it is valuable to be able to quantify the impacts of individual access points, it is equally important to study the synergistic effects of multiple driveways as a function of their frequency and spacing.
V. RESEARCH OBJECTIVE
The objective of the research will identify the expected operational impacts of access management treatments and therefore, the operational effects of one or more access points along an urban street segment. The research will also identify a methodology for estimating operational improvements as a result of access management implementation (e.g., travel time information, micro-simulation, etc).
To satisfy these objectives, the following tasks are anticipated:
- 1. Literature review of existing studies;
- 2. Survey instrument development and administration to identify further studies;
- 3. Development of a matrix based upon Tasks 1 and 2 that identifies expected operational impacts for combinations of access management treatments based upon existing studies. Factors to consider would include the effect of access management treatments on through-vehicle speed, travel time reduction, through-lane use, platoon structure, etc.;
- 4. Develop Interim Report on findings of Tasks 1 through 3, and allow one (1) month for panel review time;
- 5. Develop approach/methodology for collecting field data and/or performing micro-simulation to fill the gaps in operational understanding in the Task 3 matrix. This will include identifying a wide variety of access management configurations that can be evaluated based on the factors identified previously;
- 6. Develop methodology (possibly as a transferable look-up table) of expected operational impacts for given combinations of access management treatments, and volume range;
- 7. Document research results in final report and allow three (3) months for panel review; and
- 8. Disseminate the information to the transportation community in the form of a written document and other press releases.
VI. ESTIMATE OF PROBLEM FUNDING AND RESEARCH PERIOD
It is estimated that the research will require funding of approximately $450,000 and 32 months to accomplish.
VII. URGENCY, PAYOFF POTENTIAL, AND IMPLEMENTATION
The results can be used by transportation professionals at all levels of government to identify the expected operational benefits of different combinations of access management treatments. It will also provide a transferable methodology for identifying operational impacts. The results will allow practitioners a methodology to predict operational impacts for a given combination of access management treatments and traffic volumes, thus providing a methodology to choose between design alternatives in a systematic manner.
There is urgency in the U.S. to provide alternatives in design that improve operations of the overall roadway network. By understanding the operational impacts of access management treatments, states and municipalities can develop guidelines to better implement access management in design that will improve overall roadway capacity. The payoff potential could be great as roadways are designed to accommodate higher volumes of traffic without the need to increase overall lane miles.
Sponsoring Committee: AHB70, Access Management
Source Info: TRB Highway Capacity Manual Committee
TRB Access Management Committee
Date Posted: 09/12/2007
Date Modified: 12/02/2008
Index Terms: Access control (Transportation), Arterial highways, Medians, Unsignalized intersections, Case studies, Driveways,