Road Geometric Design Research Articles

Drainage Considerations for Porous Asphalt Surface Course Design

The drainage performance of a porous asphalt surface course depends on the drainage properties of the asphalt mixture as well as the geometric design of the individual road sections. Hence, the minimum thickness requirement may vary from one section to another in a single road project because of changes in road geometric design, even though the same asphalt mixture is used for the surface course. However, the influence of road geometric properties and the effects of longitudinal gradient and cross slope on the thickness design of the surface course have not received the deserved attention. No readily available design tools allow highway engineers to effectively perform design checks on thickness requirements. Convenient plots were developed to meet this need. A three-dimensional finite element program was used to study the effects of cross and longitudinal slopes on the drainage performance of the porous asphalt surface course. Both the longitudinal gradient and the cross slope of a road section significantly affect the drainage capacity of the porous surface course. A family of thickness-requirement graphs was prepared for easy application. These graphs were plotted as functions of design rainfall, thickness of surface course layer, width of pavement, and longitudinal and cross slopes.

Approximate Perspective Design of Roads

Coordination of horizontal and vertical alignments is an important principle in road geometric design, and it should begin with preliminary design. Perspective drawings and sketches are considered to be effective means for evaluating this coordination. In the present paper, an approximate method is developed for the quick and adequately accurate drawing of the perspective of a road, and a relevant example is given. The method is based on the use of vanishing points and the properties of a equal-tangent parabola. It is assumed that the spatial curve of the centerline of a road element—defined as a road section along which the horizontal and vertical curvatures are constant—is an equal-tangent parabola. The proposed method does not substitute computer aided drawing of the perspective, which is essential during the detailed design stage. It is nevertheless judged to be useful and more appropriate during the preliminary design stage, for it allows an engineer, designer, or researcher to reliably trace the perspective view of various horizontal and vertical alignment combinations and to obtain the desired coordination.

Driving Dynamic Aspects and Related Safety Issues for Modern Geometric Design of Non-Built-Up Roads

Safety issues affecting modern highway geometric design of roads in non-built-up areas are discussed. The status of knowledge of Safety Criteria I and II (achieving operating speed consistency and design consistency) for two-lane rural roads is briefly reviewed. Considerations for design consistency are also extended to multiple-lane rural and suburban road design to evaluate good, fair, and poor design levels for these road categories. Utilization ratios (n) for maximum permissible side friction factors are developed for different road categories, topography levels, and maximum and minimum superelevation rates. Relevant minimum radii for curve design are established and compared with present AASHTO values insofar as possible. Safety Criterion III (achieving driving dynamic consistency) is further developed, through which reliable and quantitative ranges between the side friction assumed and the actual friction demand at curved sites could be established. AASHTO's design policy is evaluated on the basis of Criteria II and III. The evaluation showed, for lower design speeds (30, 40, and 50 mph), an unbalanced degree of curve and superelevation rate ranges, which lead to poor design practices from a safety viewpoint. By heeding the three safety criteria and the recommended side-friction utilization ratios, sound alignments can be established for the various design cases in future highway geometric design and redesign and for resurfacing, restoration, and rehabilitation strategies.

Driving Dynamic Aspects and Related Safety Issues for Modern Geometric Design of Non-Built-Up Roads

Safety issues affecting modern highway geometric design of roads in non-built-up areas are discussed. The status of knowledge of Safety Criteria I and II (achieving operating speed consistency and design consistency) for two-lane rural roads is briefly reviewed. Considerations for design consistency are also extended to multiple-lane rural and suburban road design to evaluate good, fair, and poor design levels for these road categories. Utilization ratios (n) for maximum permissible side friction factors are developed for different road categories, topography levels, and maximum and minimum superelevation rates. Relevant minimum radii for curve design are established and compared with present AASHTO values insofar as possible. Safety Criterion III (achieving driving dynamic consistency) is further developed, through which reliable and quantitative ranges between the side friction assumed and the actual friction demand at curved sites could be established. AASHTO's design policy is evaluated on the basis o...

Reliability indices for road geometric design

Highway engineers, when asked to state the safety of a particular design, are usually at a loss to give a single meaningful measure as is possible in structural or geotechnical engineering. This paper outlines a method to estimate the margin of safety and reliability index for isolated highway components. The stopping sight distance is used to demonstrate the method. The method uses the basic highway design equations. On the assumption that the variables are random, the expected value of the mean and the variance are estimated; and from these the margin of safety and the reliability index are calculated. The most likely combination of variables for the existing design condition may also be estimated. The variables included represent the characteristics of the driver, the vehicle, and the road surface.A method is proposed to specify the design parameter's value representing a road's strategic importance, the users, the vehicles, the drivers, the environment, the terrain, and the standard of design and construction. The apparent advantage of the proposed reliability-based method is that the designer must explicitly specify the importance of the modifying factors and may also more closely investigate the behaviour of the variables in the design parameters in the critical region near noncompliance. Key words: limit states design, stopping sight distance, safety, highway design, reliability.

The geometric design of roads

The geometric design of roads

Statistiek en verkeer*

SummaryStatistics and traffic.Traffic defined as the collectivity of moving (andparked) vehicles, is the direct consequence of transportation needs as they occur in society as a result of economic and social activities. Hindrances to the free flow of traffic result in decrease and even stagnance of human activities. The correct functioning of traffic facilities is imperative to modern human society.To solve traffic engineering problems, present and future traffic loads must be determined and traffic behaviour in relation to geometric road design must be known. As the volume and character of human activity fluctuates and changes in time, traffic loads will vary in volume and character. Any parameter used to indicate traffic load (volume, speed, etc.) has no well determined value but rather should be evaluated by describing its frequency distribution (histogram). The traffic engineer should not design for the average value, because the facility would not render the expected services whenever a value higher than average would occur. Nor should he design for the highest value, because then the facility will be underloaded, and therefore uneconomical, except when this exceptionally high value occurs. This is illustrated in case of determining the “design speed” for a curve in a road and of determining the “design‐volume” for a road‐crosssection.To determine the histograms of the various design‐factors numerous observations of actual traffic are required. The survey programs should be well correlated with the data which are required for designwork to limit scope and cost. Often, sampling will render data of sufficient accuracy; the required accuracy being set by the design engineer. The statistician will render essential services in determining the size of samples, the survey programs and supervising the correct processing of data.The statistical problems involved in traffic‐behaviour research work are even more challenging because many of the factors involved are not known and should be detected through large series of observations.