2013 Session: 727

• Modeling the Relationship Between the Radius and Superelevation in Horizontal Curve Design
  Abstract: The paper presents a methodology for correlating the radius of horizontal curve (R) and the superelevation rate (e) when the radius is above the minimum. The method proposed, implements the radial utilization friction factor (fR) concept. The relationship between e and R was established by assuming linear or parabolic relationship between the superelevation rate and the radial friction coefficient and generating e-fR distribution factors (ãL, ãP) correspondingly. The proposed models are sensitive to the superelevation policy by applying the desired maximum superelevation rate that is appropriate with the design speed. The results were compared to the e-R relationships recommended by the United States policy on geometric design of highways (AASHTO): method 5 (for high speed facilities) and method 1. Both models generated intermediate radii results between AASHTO method 5 and method 1. The highest difference between AASHTO method 5 and the proposed models and between AASHTO method 1 and the linear model occurs when the radius reaches its maximum value for a minimum superelevation (emin=2%). The linear model resulted in lower radii design values than the parabolic model and is therefore superior in terms of topography harmonization and right of way.
  Authors: Bassan, Shy
  Authors: Bassan, Shy
  Year: 2013
  Document Type: Paper
  Subject: Design; Safety and Human Factors
  Session: 727
  Paper Number: 13-0367
• Modeling Operating Speed using Traffic, Posted Speed Limit, and Geometric Characteristics
  Abstract: Designers recognize operating speed as a measure of roadway consistency and driver expectancy. By assessing operating speed, designers have an opportunity to assess the expected speed of individual vehicles traversing successive roadway elements. While most previous studies focused on how horizontal curvature affects free-flow speed, several other factors can influence vehicle operating speed. This paper, using speed data collected on tangent section of rural freeways and rural highways in Texas (two-lane and four-lane), determines the relationship between operating speed, posted speed limit, and other selected independent variables. To account for the effect of geometrics and traffic characteristics, researchers developed speed prediction equations. The significant variables for four-lane, limited access freeways are posted speed limit, number of ramps within 3 miles, percent trucks, lane (inside or outside), median width, left shoulder width, and vehicle type (car or truck). The significant variables for four-lane, non-limited access highway are posted speed limit, downstream horizontal curve angle, lane (inside or outside), median width, left shoulder width, right shoulder width, vehicle type (car or truck), and light level (day or night). The significant variables for two-lane, non-limited access highway are posted speed limit, number of access points within 1,000 ft, number of horizontal curves within 1 mile downstream of site, number of horizontal curves within 1 mile upstream of site, lane width, vehicle type (car or truck), and light level (day or night). These findings provide insight into how select variables affect operating speeds.
  Authors: Fitzpatrick, Kay; Robertson, James; Park, Eun Sug; Iragavarapu, Vichika; McDaniel, Darren
  Authors: Fitzpatrick, Kay; Robertson, James; Park, Eun Sug; Iragavarapu, Vichika; McDaniel, Darren
  Year: 2013
  Document Type: Paper
  Subject: Design; Safety and Human Factors
  Session: 727
  Paper Number: 13-0391
• Extension of the AASHTO Model for Horizontal Sightline Offset to Apply to Beginnings and Ends of Simple Horizontal Curves.
  Abstract: Sight distance is an important element of design of curved sections of highways. To ensure that design sight distance is provided on curved sections, the AASHTO guideline requires that insides of horizontal curves be cleared of obstructions to sight. However, the guideline’s analytical method for determining the extent of clearance is suitable only for middle sections of horizontal curves that are longer than sight distance. The guideline suggests use of graphical or computational methods for determining offsets for cases its model is inapplicable. The graphical method is tedious and difficult to integrate into design computer models. The computational method suggested is useful only for circular curves that are longer than sight distance but the mathematical background used to develop it is lacking hence its analytical aspects are unknown. This study developed a new analytical model for clearance offsets for sections that are not covered by the AASHTO model. Model development started with deriving equations for a roadside spiral curve which acts as a boundary for the area that accommodates sightlines on insides of curves. Main factors in the equations are driver location, sight distance, and curve radius. Equations for offsets were then derived as ordinates from horizontal alignment to the spiral curve. Design charts were then developed for short curves and for long curves to work with the existing AASHTO model for maximum offset. The results of this study will be of interest to engineers undertaking new designs, improvement designs for sites with limited sight distance, and maintenance.
  Authors: Mauga, Timur
  Authors: Mauga, Timur
  Year: 2013
  Document Type: Paper
  Subject: Design; Safety and Human Factors
  Session: 727
  Paper Number: 13-0558
• Safety Evaluation of Traffic Operation Environment in Freeway Tunnels
  Abstract: It is obvious that the operation safety in freeway tunnel is significant, after the tremendous freeway tunnel accidents in China over the past decade. With the aim to evaluate the safety of freeway tunnel operation, a matter-element and extension evaluative (AEE) model on the basis of the extenics is proposed. As the evaluation indicators are changeable, it is critical to compose risk indicators by a large evaluation indicator system. The Delphi Method (DM) is used to execute a sensitivity analysis of the indicator system, and the AHP method is used to determine the weight of the evaluation index system, considering the experts¡¯ knowledge and experience as well as the data itself implies. At the end, the value of eigenvalue in the freeway tunnel operating environment is obtained, the results indicate that the indicator system and weights are reasonable. The research results can provide a theoretical guidance for the analysis and evaluation of the tunnel operating environment.
  Authors: ZHOU, Jibiao; CHEN, Hong; LI, Xiaowei; ZHANG, Long; GAN, Zuoxian
  Authors: ZHOU, Jibiao; CHEN, Hong; LI, Xiaowei; ZHANG, Long; GAN, Zuoxian
  Year: 2013
  Document Type: Paper
  Subject: Design; Safety and Human Factors
  Session: 727
  Paper Number: 13-1274
• Roadside Clearance Limit on Horizontal Curves with Transition Arcs:Sites with Circular Arcs Shorter than Sight Distance
  Abstract: The AASHTO guideline for geometric design of highways requires that insides of horizontal curves be cleared of obstacles to vision in order to provide required sight distance. However, the guideline does not have a method for determining the extent of roadside clearance for horizontal curves having transition arcs. Instead, the guideline recommends use of graphical and computational methods for the purpose. The graphical method is tedious and prone to errors. The computational method works only for sites with circular arcs equal to or longer than sight distance. This study has developed an analytical model for design clearance offsets for sites with circular arcs shorter than sight distance. Offsets were defined as normal ordinates from highway alignment to a curve that demarcates roadside area that accommodates sightlines. Variables in the model are sight distance, lengths of transition and circular curves, radius of circular curve, and driver location. Using the model, a design chart for maximum offsets and another for intermediate offsets were developed for use by practitioners. The maximum and intermediate design offsets for horizontal curves that have circular arcs that are shorter than sight distance were found to be significantly shorter than those for circular curves that are longer than sight distance. Use of the design offsets will avoid extra excavation costs that result from using bigger offsets determined by other models. Results of this study will be of value to engineers involved in design policies, highway maintenance as well as improvement designs for sites deficient in sight distance.
  Authors: Mauga, Timur; Ghanma, Mwafag; Ahmed, Kamran
  Authors: Mauga, Timur; Ghanma, Mwafag; Ahmed, Kamran
  Year: 2013
  Document Type: Paper
  Subject: Design; Safety and Human Factors
  Session: 727
  Paper Number: 13-3113
• Homogeneous Road Segment Identification Based On Inertial Operating Speed
  Abstract: Geometric design consistency evaluation is an important tool for the road safety evaluation of a road alignment. This evaluation determines the grade of fitting between road behavior and drivers’ expectations. Therefore, road consistency and road safety are related: a high consistency level leads to lower crash rates.While local consistency models only focus on a road section, such as curves or tangent-to-curve transitions, global consistency models consider the whole road segment. They evaluate the dispersion of the road segment operating speed profile, assuming that highly variable operating speed profiles increase the probability of accident occurrence.The result of those methodologies varies depending on the selection of the road segment. They should be applied to homogeneous road segments. Some methodologies are already established for splitting the road into homogeneous segments, but they are only focused on some geometric or traffic related variables.A new performance-based methodology for determining road homogeneous segments is presented. It is based on a new variable: the inertial operating speed, defined as the 1,000 m moving average value of the operating speed. It is an indicator of the behavior of drivers during last 1,000 m, allowing its comparison with the local requirements and, therefore, allowing a road consistency evaluation. Identifying significant minimum points located along inertial operating speed profile allows determining homogeneous road segments, using those points as limits between segments.
  Authors: Garcia, Alfredo; Llopis-Castello, David; Perez-Zuriaga, Ana Maria; Camacho-Torregrosa, Francisco Javier
  Authors: Garcia, Alfredo; Llopis-Castello, David; Perez-Zuriaga, Ana Maria; Camacho-Torregrosa, Francisco Javier
  Year: 2013
  Document Type: Paper
  Subject: Design; Safety and Human Factors
  Session: 727
  Paper Number: 13-3545
• A new Set of Guidelines for Road Diet Conversions
  Abstract: Road diets, which convert four-lane highways to three-lane cross sections, are an innovative solution to address mobility and safety concerns under budgetary constraints. These improvements can assist in the development of multimodal corridors with minimal impact on automobile mobility, while retaining the original right of way. Past research has focused on evaluating road diet safety, but minimal guidance exists on determining when such conversions are appropriate from an operational perspective. The proposed guidelines focused on evaluating and comparing the operation of three- and four-lane roads at signalized intersections to provide basic guidance as to when the road diet conversion is appropriate. One of the important findings of this research is the expansion of the usable range for road diets. Prior experience has limited road diet application to roadways with ADTs less than 17,000 vehicles per day. This research identifies the importance of side street volumes and supports the utilization of road diets on roadways with volumes up to 23,000 vehicles per day. This paper provides comprehensive guidance for road diet evaluation including operational performance, correctable safety problems and identifies a list of evaluation elements that should be examined when in-depth analysis of alternatives is required.
  Authors: Stamatiadis, Nikiforos; Kirk, Adam
  Authors: Stamatiadis, Nikiforos; Kirk, Adam
  Year: 2013
  Document Type: Paper
  Subject: Design; Safety and Human Factors
  Session: 727
  Paper Number: 13-1321
  Practice-Ready: Yes
• A new Set of Guidelines for Road Diet Conversions
  Authors: Stamatiadis, Nikiforos
  Authors: Stamatiadis, Nikiforos
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Design; Safety and Human Factors
  Session: 727
  Paper Number: 13-1321
• Safety Evaluation of Traffic Operation Environment in Freeway Tunnels
  Authors: ZHANG, Long
  Authors: ZHANG, Long
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Design; Safety and Human Factors
  Session: 727
  Paper Number: 13-1274
• Modeling Operating Speed using Traffic, Posted Speed Limit, and Geometric Characteristics
  Authors: Fitzpatrick, Kay
  Authors: Fitzpatrick, Kay
  Year: 2013
  Document Type: Presentation; Poster
  Subject: Design; Safety and Human Factors
  Session: 727
  Paper Number: 13-0391