Development of Geometric Design Guidance for Metered On-ramps
Ramp metering places a traffic signal at an on-ramp to manage traffic demand entering a freeway. By
doing so, overloading of the downstream freeway may be prevented, and in turn, mainline congestion
is delayed or avoided, and the total delay for freeway and on-ramp traffic is reduced. Such benefits of
ramp metering have been well documented. In 2000, the Minnesota Department of Transportation
turned off more than 400 ramp meters in the Minneapolis-St. Paul area. The results were: the freeway
throughput decreased by 9%, travel time increased by 22%, speeds dropped by 7%, and crash
increased by 26%. Recent studies conducted in the San Francisco Bay Area also indicated that ramp
metering reduced the travel time by 30% along an 18 mile section of Route 580. In view of the ever-increasing
difficulty of adding more capacity to the existing freeway system, ramp metering has
become an important traffic management tool to combat traffic congestion and improve system
performance. By the end of 2011, approximately 3900 ramp metering locations have been
implemented in over 30 metropolitan areas across more than 15 states.
Ramp metering has profound impacts upon the operations and design of an on-ramp. When an on-ramp
is metered, all on-ramp traffic has to stop and queue upstream of the limit line before
accelerating and merging with mainline traffic. For a ramp meter to function without adversely
impacting the upstream feeding roadway operations, the queue created by the ramp meter must be
properly managed, be it contained within the on-ramp, or at the upstream feeding facilities. If there is
not sufficient storage the result is that ramp meters have to operate constrained with the available on-ramp
storage capacity. When on-ramp queue overspill is impending, ramp meters have to cycle faster
or even stop metering, losing the capability of considering how mainline or the entire system is
performing thereby degrading the operation of the system. In addition, due to all vehicles having to
come to a full stop, a longer acceleration distance must also be designed to accommodate the zero
starting speed at the limit line.
Currently, there is no well-accepted geometric design guideline that considers the necessary additional
storage length and acceleration distance for metered on-ramp operations.
The objective of the research
is to develop design guidance to size on-ramp queue storage areas and acceleration distance for new or
reconstructed on-ramps with ramp meters.
The proposed research topic is of national significance. The conclusions from this research will bridge
the gap of knowledge in terms of metered on-ramp geometric design guidance. The findings will
provide a foundation for AASHTO to provide guidance on metered on-ramp geometric design in its
“A Policy on Geometric Design for Streets and Highways”, which will in turn help streamline the
consideration of ramp metering in on-ramp design across the nation. In practice, properly sized storage
and acceleration lane length at metered on-ramps will lead to safer and more efficient ramp metering
operations. The tort liability potential at metered on-ramps with insufficient storage and acceleration
lane length may be reduced, if not eliminated.
Each year, many on-ramps are modified or reconstructed in the more than 50-year old interstate system across the nation. Unfortunately, most of the on-ramps were reconstructed as much as 50 years ago; the possibility of future ramp metering was not evaluated, the geometric provisions for ramp metering went uninstalled, and the opportunity to improve future system performance was lost. Again and again, ramp metering has been proven to be an effective means to improving freeway system performance.
A literature review revealed few comprehensive studies on on-ramp queue storage and acceleration
distance design at metered on-ramps. The highway geometric design policy published by the American
Association of State Highway and Transportation Officials (AASHTO) is silent on metered on-ramp
design. Guidelines from some Departments of Transportation (DOT), such as the Wisconsin DOT, do
specify storage needs for metered on-ramps, but no supporting studies were presented. In the
Intelligent Transportation System Design Manual, the Wisconsin DOT specifies that an on-ramp must
provide storage length for a minimum of 10% of the current peak hour volume to ensure that ramp
meter queue does not spill back into surface streets. This factor is vital in determining whether the
ramp will contain one or two Single Occupancy Vehicle lanes. The manual also specifies that on-ramps should accommodate a minimum of 10% of the design year projected peak hour volume for
reconstruction case; and for ramp meters retrofitted to existing conditions, a minimum storage of 5%
of the current peak hour volume may be used with additional approval.
Researchers at the Texas Transportation Institute (TTI) recommend the optimum distance from center
of upstream signalized intersection to ramp meter is about 800 ft., including 250 ft. of minimum
stopping sight distance. The following relationship was proposed to determine single lane storage
L = 0.25V - 0.00007422V^2
Where, L = storage length, m; V = peak hour demand, vehicles per hour (vph); and V ≤ 1600 vph.
The empirical relationship was developed based on the special freeway / frontage road configuration in
Texas. As the ramp length is generally limited by the separation between freeway and frontage road, it
is understandable why the equation determines a storage length of only about 18 passenger cars even
when the volume is 1600 vph.
In early 2012, the Caltrans Division of Research and Innovation retained a consultant to perform a
preliminary investigation on “Ramp Metering Design”. The preliminary investigation report was made
available online: http://www.dot.ca.gov/newtech/researchreports/preliminaryinvestigation/docs/rampmeteringdesignpi_2-24-2012.pdf. Based on the scan of state, national and international
guidance and specifications documents, together with the synthesis of relevant research projects, the
consultant concluded that: “we did not uncover ramp meter design guidance exceeding the level of
detail in Caltrans’ own specifications or explicit findings that spelled out design solutions for new or
retrofit metered ramps.” However, in the Caltrans Ramp Meter Design Manual dated January 2000,
there is no specific guidance on how to size on-ramp queue storage length for new or reconstructed on-ramps.
In addition, the acceleration distance specified was based on some limited acceleration
performance data of one passenger car some 20+ years ago.
The preliminary investigation also pointed out that “The trend in research, particularly in recent years,
appears to focus on traffic detection, control strategies, queue management and ramp network
management rather than on the design characteristics.” It is therefore important to develop design
guidance for metered on-ramps for better ramp metering operations.
Review existing guidance and relevant research
Develop a work plan to collect the necessary data to address identified gaps
Execute work plan
Document project activities in research report
Develop new/revised design guidelines
New or revised material in the AASHTO "Green Book" on guidance for metered on-ramp geometric design. Support for similar material to be included in state-level design guides.
By the end of 2011, approximately 3900 ramp metering locations have been
implemented in over 30 metropolitan areas across more than 15 states. Each year, many on-ramps are modified or reconstructed in the more than 50-year old interstate system across the nation. Unfortunately, most of the on-ramps were reconstructed as much as 50 years ago; the possibility of future ramp metering was not evaluated, the geometric provisions for ramp metering went uninstalled, and the opportunity to improve future system performance was lost. Again and again, ramp metering has been proven to be an effective means to improving freeway system performance, and designers in State DOTs and their design consultants will benefit from this specific guidance.
|Sponsoring Committee:||AKD10, Performance Effects on Geometric Design
|Research Period:||12 - 24 months|
|RNS Developer:||Zhongren Wang, Lester Lee (Caltrans)|
|Source Info:||Based on discussions during the 2013 mid-year meeting of Transportation Research Board (TRB) committees AFB10 and AHB65, along with the American Association of State Highway and Transportation Officials (AASHTO) Technical Committee on Geometric Design, in Irvine, CA in July 2013. Also based in part on activities and discussions between researchers and panel members in National Cooperative Highway Research Program (NCHRP) Project 3-105, "Design Guidance for Interchange Loop Ramps". |
Contract to a Performing Organization has not yet been awarded.
|Index Terms:||Geometric design, On ramps, Ramp metering, Guidelines, Minneapolis (Minnesota), Saint Paul (Minnesota), Minnesota Department of Transportation, Highway traffic control, Acceleration (Mechanics), |
Operations and Traffic Management