Zhongren Wang

Queue Storage Design for Metered On-Ramps

Queue storage at a metered freeway on-ramp is an essential design element for metered on-ramps to prevent on-ramp queue from extending beyond on-ramps. In this paper, various existing methodologies that are used to size the queue storage at metered on-ramps were first reviewed. It was found that queue storage sized using 7% of peak hour on-ramp demand is widely accepted in practice. A limited dataset collected in California helps provide some reality check for this method. It is recommended that queue storage be recognized as an indispensable design element at metered on-ramps, and detailed sizing guidance be developed in the highway geometric design policy of the American Association of State Highway and Transportation Officials (AASHTO).

Placement Design of Ramp Control Signals

Some current design guidelines specify that standard-mounted ramp control signals be placed almost aligned with the limit line. As a result, signal indications are typically placed outside of a drivers comfortable cone of vision, as required by the Manual on Uniform Traffic Control Devices (MUTCD). In this paper, the placement design of ramp control signals was analytically examined in relation to the satisfaction of a drivers comfortable cone of vision for stopped vehicles at the limit line and the satisfaction of stopping sight distance for approaching vehicles in accordance with MUTCD. Relationships between location of limit line, location of signal standard, ramp geometry, and approaching speeds were derived. Sample lookup design charts were also developed to facilitate the development and evaluation of signal placement design. A brief analysis using these relationships concluded that signal standards placed in alignment with the limit line would violate not only the comfortable cone of vision of stopped drivers but also the stopping sight distance of approaching vehicles. For a loop on-ramp with a 300-ft radius, standard-mounted signals on the left side of an on-ramp should be placed at least 22 ft downstream of the limit line to satisfy the requirements of both the stopped and approaching vehicles. In contrast, signals on the right side could satisfy only the stopped vehicles but not the approaching vehicles if placed at least 44 ft downstream of the limit line. (Left and right here are relative to the direction of travel.) Therefore, signals placed on the left side of the on-ramp curve (even with a radius of 500 ft) are more critical than those on the right side, especially when the approaching stopping sight distance is important.

Ramp Metering Status in California

The purpose of this paper is to provide an update of the major improvement in terms of ramp metering design and operations in California. These updates include ramp metering policies, ramp metering development plans, ramp metering design manual, and ramp metering and system management initiatives.

Deriving Sight Distance on a Compound Sag and Circular Curve in a Three Dimensional Space

Insufficient roadway sight distance (SD) may become a contribution factor to traffic collisions or other unsafe traffic maneuvers. The sight distance (SD) for a two-dimensional (2-d) sag or circular curve has been addressed in detail in various traffic engineering literatures. Although three-dimensional (3-d) compound sag and circular curves are often found along ramps, connectors, and mountain roads, the sight distances for these compound curves are yet to be analyzed on an exact analytic setting. By considering human-vehicle-roadway interaction, the formulas for computing the SD on a 3-d curve are derived the first time on a unified analytic framework. The 2-d sag curve SD can also be deduced from these derived formulas as special limiting cases. Practitioners can easily program these formulas or equations on a user-friendly Microsoft Excel spread sheet to calculate 3-d SD on most roadways with roadside clearance. This framework can be extended to estimate SD on roadways with obstacles partially blockin...

Determination of Onramp Weaving Length for Resolving Merging Dilemma

It is physically known that onramp merging may turn out to be difficult if the onramp (weaving/merging) length is too short because a driver under certain driving circumstances may find that either merging ahead or merging behind a neighboring vehicle on the adjacent highway lane cannot be completed. Various existing guidelines or design manuals provide no clear physical understanding and explanations to the design onramp weaving length but often based on evolved empirical experiences. By integrating human factors, vehicle dynamic characteristics, roadway surface condition, and the onramp weaving design into a single unified analytic framework, the onramp length required for a driver to merge into the highway traffic successfully is determined exactly with formulas and physical solutions to avoid the merging dilemma and enhance driving safety at highway interchanges. The design onramp weaving length is examined and evaluated with various foreseeable merging scenarios and physical examples. This analytic f...

Determination of Sight Distance on a Combined Crest and Circular Curve in a Three Dimensional Space

The sight distance (SD) on a two-dimensional (2-d) curve, namely, a vertical curve or a horizontal curve, has been well understood and documented for roadway geometric design in literature. In reality, three-dimensional (3-d) curves can be found along ramps, connectors, and often mountain roads. The sight distance on these 3-d curves, which may vary with drivers location, has not been tackled in literature on an exact analytic setting. By integrating human-vehicle-roadway interaction, the formulas for computing the SD on a 3-d curve are derived the first time on an analytic framework. The crest curve SD that has been used in various literatures, can be deduced from these derived formulas as special limiting cases. Practitioners can easily apply theses user-friendly formulas or equations on a Microsoft Excel spread sheet to calculate 3-d SD on a roadway with sufficient roadside clearance. In addition, this framework can be extended easily to cope with various scenarios in which obstacles partially blockin...

Feasibility of Using a Constant Acceleration Rate for Freeway Entrance Ramp Acceleration Lane Length Design

AbstractWhen estimating the required acceleration length of vehicles accelerating from a stopped position, a constant acceleration rate is sometimes assumed for the sake of simplicity. Nevertheless...

Recommendations for Acceleration Lane Length for Metered On-Ramps

AASHTO’s A Policy on Geometric Design of Highways and Streets (Green Book) is currently used by most state departments of transportation in the United States in determining the design length of acceleration lanes of metered on-ramps; however, the recommended acceleration lengths have not been updated for several decades. This study aimed to develop a method for determining acceleration lengths at metered on-ramps. Vehicle location versus time information was collected via parallel cameras at seven metered on-ramps in California; then, a piecewise constant acceleration model was proposed to calculate the spot speeds of individual samples at predetermined locations. The percentile distance-versus-speed profiles at each ramp were built, and regression models were generated to predict the required acceleration length at a given merge speed. The 85th percentile data were recommended as the minimum acceleration length to accommodate most drivers in accelerating to a safe merging speed. The new recommendation was compared with the existing guidance in the Green Book. On the basis of 1,658 individual samples, it was found that the recommended acceleration lengths were shorter than the Green Book guidelines by 10% to 35%. Also, results showed that acceleration lengths for tractor trailer trucks were approximately 60% greater than the Green Book guidelines.

The Sight Distance Issues with Retrofitted Single-Lane HOV Facilities

It is well-known that obstruction inside a highway horizontal curve will lead to impaired sight distance. Highway alignment design standards in terms of the minimum horizontal curve radius are specified to allow for adequate stopping sight distance at given design speeds. For a single-lane HOV facility, inside curve obstruction may occur no matter when the facility curves to the left (per travel direction) or right. A unique situation that calls for special attention is that the adjacent mixed-flow lane traffic, once queued, may become sight obstruction. Calculations indicated that such obstruction may govern the minimum curve radius design as long as the left shoulder is not less than 0.92 m, when the HOV lane is contiguous to the mixed-flow lanes. Such governance may necessitate design speed reduction, horizontal and cross-section design adjustment, or both.

Estimate Freeway Exit Sign Locations

ABSTRACT Considering that a driver decides to exit a highway upon seeing the guide sign upstream of an exit, subsequently the driver in an inside lane or the middle lanes must move onto the outside lane prior to exiting. The concern is whether the driver can accomplish this task safely and smoothly. It is apparent that an upstream exit sign cannot be placed too close to an exit or too far beyond several exits upstream. The MUTCD recommends that the sign should be placed 1 mile and 2 miles upstream of an exit without explaining the reasons for selecting the 1 mile distance. By integrating driver decisions, vehicle acceleration characteristics, tire-road traction into a single analytic framework, the location upstream of an exit where an exit sign should be installed is determined for a driver to get off at the right exit successfully. Practitioners can easily apply these user friendly formulae and equations derived from the framework to compute the required distance ‘D’ between a highway exit and an upstream exit sign for guiding drivers to exit the highway safely. Additionally, parameters for these formulae can be adjusted to resemble various exiting scenarios.