Said M. Easa

Selection of roadway grades that minimize earthwork cost using linear programming

Abstract Roadway grades are normally established to satisfy the geometric specifications of the road. Once the grades are established, the earthwork allocations that minimize the cost are determined. This paper presents a model that links these two activities. The model selects the roadway grades that minimize the cost of earthwork and satisfy the geometric specifications. The geometric specifications pertain to the elements of the vertical alignment, elevation of grade line at specified stations, horizontal and vertical alignment relations, and type of vertical curve. The model enumerates all technically feasible grades and solves the linear programming problem to minimize earthwork allocation cost for those grade alternatives that satisfy borrow pit and landfill capacity constraints. The model incorporates important earthwork details and guarantees obtaining the global minimum earthwork cost. Application of the model is illustrated by a numerical example and model extensions to accomodate other design and construction aspects are presented.

Analytical Model for Sight Distance Analysis on Three-Dimensional Highway Alignments

Existing sight distance models are applicable only to two-dimensional (2-D) separate horizontal and vertical alignments or simple elements of these separate alignments (vertical curve, horizontal curve). A new model is presented for determining the available sight distance on 3-D combined horizontal and vertical alignments. The model is based on the curved parametric elements that have been used in the finite element method. The elements presented are rectangular (4-node, 6-node, and 8-node elements) and triangular. These elements are used to represent various features of the highway surface and sight obstructions, including tangents (grades), horizontal curves, vertical curves, traveled lanes, shoulders, side slopes, cross slopes, superelevation, lateral obstructions, and overpasses. The available sight distance is found analytically by examining the intersection between the sight line and the elements representing the highway surface and the sight obstructions. Application of the new model is illustrate...

PASSING SIGHT DISTANCE ON TWO-LANE HIGHWAYS: REVIEW AND REVISION

Several models have been developed to determine the minimum passing sight distance required for safe and efficient operation on two-lane highways. The American Association of State Highway and Transportation Officials has developed a model assuming that once the driver begins a pass, he/she has no opportunity but to complete it. This assumption is believed to result in exaggerated passing sight distance requirements. Considerably shorter passing sight distance values are presented in the Manual of Uniform Traffic Control Devices and are used as the marking standards in Canada and the U.S.A. More appropriate models have been developed considering the drivers opportunity to abort the pass, and are based on a critical sight distance which produces the same factor of safety whether the pass is completed or aborted. However, these models need to be revised to determine the passing sight distance requirements more accurately and to closely match field observations. In this paper, a revised model for determining the minimum required passing sight distance was developed, based on the concept of critical sight distance and considering the kinematic interaction between the passing, passed, and opposing vehicles. The results of the revised model were compared with field data and showed that the revised model simulates the passing manoeuvre better than the currently-available models which are either too conservative or too liberal. The results showed that the passing sight distance requirements recommended in the Manual of Uniform Traffic Control Devices are sufficient at low design speeds (50-60 k.p.h.) and for manoeuvres involving passenger cars only. For higher design speeds, the Manual of Uniform Traffic Control Devices standards are less than the passing sight distance required for safe and comfortable passes. The deficiency was found to increase with the increase in design speed, and reaches about 36% at a 120-k.p.h. design speed. Based on these results, major revisions to the current Manual of Uniform Traffic Control Devices marking standards are recommended.

Analysis of commercial mini-bus accidents.

This paper presents a study of mini-bus traffic accidents aimed at gaining insight into the factors affecting accident occurrence and severity. Understanding these factors can help to bring forth realistic strategies to improve the safety of these buses. Two disaggregate models related to the time until accident occurrence and the number of accident injuries were specified and estimated. The models were estimated using data collected from 438 mini-bus drivers in Jordan. The estimated models yielded significant associations between the independent variables and both the time until accident occurrence (first, second and third accidents) and their corresponding number of injuries (accident severity). Higher accident rates were associated with drivers who were unmarried, took too few rest breaks and had short time intervals since previous accidents. Lower accident rates were associated with drivers who had long bus-driving and private vehicle-driving experience. The results indicate that the longer a mini-bus driver goes without an accident, the less likely it is that he will have an accident. The results also indicate that previous accident type affects the duration of the upcoming traffic accident. Greater accident severity was associated with single-vehicle accidents, rural intercity routes and speeding. Accident severity decreased and the time between two accidents increased when the previous accident was severe. The results seem to indicate that post- and immediate accident history affect the severity of upcoming accidents. Seven recommendations based on these findings are made in an attempt to improve mini-bus safety.

RELIABILITY APPROACH TO INTERSECTION SIGHT DISTANCE DESIGN

The intersection sight distance (ISD) design presented by AASHTO is based on extreme values of the component design variables such as design speed, perception–reaction time (a high percentile), and friction coefficient (a low percentile). A reliability method is presented, based on AASHTO, that does not rely on extreme values but instead considers the moments (mean and variance) of the probability distribution of each random variable. The method also accounts for correlations among the component random variables. In Cases I and II of AASHTO, the variations of the sight distance along both legs of the intersection are considered for both design and evaluation. For evaluation (involving an exiting obstruction), these variations are combined into a single variable that determines whether the corresponding sight line is obstructed. In Case III, only the sight distance leg along the major road has variations. The proposed method is straightforward and involves simple, closed-form mathematics for calculating sight distance and associated reliability. Sensitivity of ISD to various design variables is examined. ISD reliability-based values for various cases are presented from data reported in the literature, and results are compared with current AASHTO design values.

Highway Alignment: Three-Dimensional Problem and Three-Dimensional Solution

Highway geometric design has usually been considered in separate two-dimensional (2-D) projections of horizontal and vertical alignments. Such a practice was followed mainly because three-dimensional (3-D) analysis of combined highway alignments was expected to be difficult. As a result, the effect of ignoring the 3-D nature of the highway alignment could not be quantified. With the long-term objective of developing 3-D design practice, a framework for 3-D highway geometric design was developed and 3-D sight distance was extensively studied as the first design basis. The status of sight distance in current design policies and previous research is summarized, and mainly 2-D analysis was considered. The five main tasks performed to cover the 3-D highway sight distance are presented. (a) As a preliminary step, the 2-D sight distance on complex separate horizontal and vertical alignments was modeled, and the finite element method was used for the first time in the highway geometric design. (b) The 2-D models were then expanded to cover the daytime and nighttime sight distances on 3-D combined alignments. (c) The analytical models were coded into computer software that can determine the available sight distance on actual highway segments. (d) The models were applied in 3-D design of combined horizontal and vertical curves in cut-and-fill sections, and preliminary design aids were derived. (e) Finally, a new concept of red zones was suggested to mark the locations on alignments designed according to current practices where the available sight distance will drop below that required. A comprehensive work on 3-D sight distance analysis has been compiled that should be of great importance for highway researchers and professionals.

Reliability-based design of sight distance at railroad grade crossings

The existing method of sight distance design at railroad crossings is deterministic. This paper presents a probabilistic method that accounts for the variations of the component design variables and their correlations. Two cases are considered. Case 1 addresses the sight distance needs along the highway and the railroad for an approaching vehicle. Case 2 addresses the sight distance needs along the railroad for a stopped vehicle. The probabilistic method is based on the advanced first-order second moment of reliability analysis. Case 1 is modeled as a system with two parallel components. The probability of failure of each component and the system probability of failure are developed. Case 2 is modeled as a single-component system, and a design graph for sight distance needs along the railroad is presented. The method is validated using Monte Carlo simulation; its application is illustrated using numerical examples; and the sensitivity of the design variables is examined. The proposed method should result in safer operations at railroad grade crossings.

Low-temperature stresses and fracture analysis of asphalt overlays

Thermal cracking of asphalt overlays is a leading cause of pavement deterioration. The thermoelastic response of a multilayered pavement structure is modeled using a transient thermal analysis followed by a quasi-static stress analysis at discrete time intervals using finite element analysis. Numerical analysis of two- and three-dimensional cracking problems is performed. Based on a fracture mechanics approach, the potential of thermal cracks to propagate through the overlay is examined using both a displacement formula and an energy-balance principle. The interaction between multiple cracks and the effect of bond between layers on crack propagation are examined. The proposed numerical methods for analysis of pavement thermal cracking provide a means to characterize and optimize different evolving materials and innovative pavement reinforcement techniques.

MODELING HEADLIGHT SIGHT DISTANCE ON THREE-DIMENSIONAL HIGHWAY ALIGNMENTS

Sight distance is one of the major elements that must be considered in the geometric design to achieve safe and comfortable highways. Daytime sight distance has been extensively studied, and analytical models for two-dimensional (2-D) and three-dimensional (3-D) alignments have been developed. However, nighttime (headlight) sight distance has received less attention. Despite the higher accident rate during nighttime than during daytime, existing analytical models evaluating headlight sight distance are very primitive. Moreover, the interaction between the horizontal and vertical alignments has not been modeled. A four-phase analytical model for headlight sight distance on 3-D combined alignments is presented. The model is an application of the finite-element technique in highway geometric design. The model can determine the maximum distance that can be covered by the vehicle’s headlights and that is not obstructed by any sight obstructions including the road surface. On the basis of this analytical model, computer software was developed and used in a preliminary application for 3-D headlight sight distances on a sag or crest vertical curve combined with a horizontal curve. The application showed that the 3-D sight distance on sag vertical curves was generally lower than the corresponding 2-D value when the sag curve was overlapping with a horizontal curve. On the other hand, the overlapping of horizontal curves with crest vertical curves enhanced the 3-D sight distance. The difference between 2-D and 3-D sight distance values in both cases increased with a decrease in the horizontal curve radius and an increase in the pavement cross slope. The model was proved to be extremely valuable in establishing 3-D highway geometric design standards.

Characteristics of the fuzzy LP transportation problem for civil engineering application

Abstract The fuzzy linear programming (FLP) transportation problem is a useful tool for some civil engineering problems. It can deal with problems whose objective and constraints are not well-defined, or information on them is not precise. It considers that the objective and constraints are fuzzy sets in the space of alternatives and defines the decision set as the intersection of the two sets. The solution is the element of the decision set which has the maximum membership grade. This paper addresses the type of problems which are suited for fuzzy LP transportation modeling; examines the formulation of the model and characteristics of the solution; and presents an example of FLP transportation formulation for the earth moving problem during highway construction. The solutions sensitivity to the fuzziness of objective and information on the quantities of supply and demand is also analysed.