A O Abd El Halim

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.

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.

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.

ASSESSMENT OF THE INFLUENCE OF COMPACTION METHOD ON ASPHALT CONCRETE RESISTANCE TO MOISTURE DAMAGE

Abstract Research work in the area of asphalt concrete stripping reflects general agreement among researches that existing adhesion theories do not completely describe the stripping phenomenon. The stripping mechanisms fail to specify the source of water necessary for stripping to start and progress with time. Locating the water source and route to the pavement interior is of great significance to effort seeking a preventive measure against stripping. This paper reports on the investigation of the influence of various construction variables focusing on compaction-induced cracks, referred to as checking, on pavement resistance to stripping. A new compaction technique was used to produce a crack-free asphalt concrete surface which offered a much higher resistance to stripping compared with conventionally compacted surfaces. Other factors associated with construction, which can influence the stripping susceptibility of compacted asphalt concrete pavements, were also considered. Cores recovered from field-compacted pavements were used throughout the investigation. The results of the experimental investigation indicated that construction has a significant influence on the compacted mix resistance to stripping. The study also identified surface water as the water source and construction cracks as the route for surface water to the pores inside the mix. Vacuum saturation, used as part of the moisture conditioning procedure, has in the past obscured the influence of construction-induced cracks on stripping susceptibility.

SHEAR PROPERTIES AS VIABLE MEASURES FOR CHARACTERIZATION OF PERMANENT DEFORMATION OF ASPHALT CONCRETE MIXTURES

Released in 1994, the Superpave® asphalt mix design system represented the culmination of a

THE PERFORMANCE OF GEOGRID REINFORCED ROAD BASES

50 million investment by the Strategic Highway Research Program to reduce the overall life-cycle costs of asphalt pavements. At this time, an extensive investigation to recommend a laboratorybased simple performance test for evaluating the resistance of Superpave mixes to permanent deformation is under way through the NCHRP. As a complementary effort, researchers at Carleton University are developing a field simple performance test for asphalt mixes known as the in situ shear stiffness test (InSiSST). InSiSST is unique to the asphalt industry; it measures the shear properties of compacted asphalt layers in the field without the need for coring or specimen preparation. Initial test results at the Superpave specific pavement studies test site (SPS-9) in Petawawa, Ontario, have shown excellent correlation between the in situ shear stiffness and observed permanent deformation. With additional testing and correlation, it is hoped that the InSiSST facility will complement the laboratory Superpave simple performance test to provide the asphalt industry with improved tools for mitigating permanent deformation. Information about the development effort is presented for context purposes, but the main objective is to further establish the benefit of fundamental asphalt shear properties for characterizing resistance to permanent deformation by presenting new and powerful performance models relating asphalt mix properties and shear properties to permanent deformation.

The influence of age on interlayer shear properties

Abstract The use of natural materials, such as straw and wood, to reinforce soil is an ancient technique. In recent years man-made materials which have high strength and are non-corrosive, were introduced to fulfil the same function. This paper reports on the results of an extensive laboratory investigation where high-strength geogrids were employed, to strenghten granular road bases subjected to traffic load conditions.

A simplified direct method for finding optimal stable trapezoidal channels

The paper focuses on the question of the influence of age on the interlayer bonding properties of two-layered specimens. The evaluation combines traditional data evaluation and artificial neural network (ANN) analysis of different data sets of pavements from Switzerland, United Kingdom and Italy. The results show that it is possible to use ANN techniques to derive models from data sets and to predict interlayer bond strength. The findings demonstrate that age has a positive effect on the interlayer bond of asphalt pavements and that long-term oven ageing can lead to similar results as in situ ageing. In addition, it was found that the positive effect of ageing is greater when a tack coat is used.

Asphalt multi-integrated rollers and steel drum compactors : Evaluating effect of compaction on permeability of asphalt pavements

An economical channel with trapezoidal cross-section is determined in this paper by considering the criterion for the side slope stability (soil conditions). This cross-section prevents the slope failure after construction. To implement this slope stability criterion, Taylors stability chart is converted to a polynomial surface equation (using multiple regression analysis) that was found to be very accurate. This equation facilitates the modelling of the optimal channel cross-section and was integrated with the flow and geometric constraints to determine the optimal channel cross-section that minimizes the total construction cost. The proposed optimization model is direct and determines the safety factor without iterations. The computer running time of the proposed model is only about 1% of the running time of the existing hybrid model. The efficiency of the proposed model, which substantially aids the conduct of sensitivity analysis, should be of interest to irrigation and drainage professionals.