Review of Fundamentals of Road Design by Wolfgang Kühn

Transportation project development is a process that involves planning and provides for the needs of various users and vehicle types while limiting environmental impacts. Road design occurs within this process in many different contexts. Examples include large-scale, new construction projects; reconstruction projects; or smaller-scale, road restoration projects. Regardless of the road design project type and context, integrating the three dimensions of horizontal alignment, vertical profile, and cross section are central to the process. This book aims to provide a review of the theoretical constructs used in developing German road design criteria, and demonstrates the application of these criteria using example problems throughout. The book also contains a review of several emerging research topics in road design, including visualization and three-dimensional design approaches. Fundamentals of Road Design is guided by the policies and procedures for rural roads in Germany. It is intended to serve as a reference text for design professionals and a course textbook for university students.

Fundamentals of Road Design is organized into 12 chapters. The first chapter introduces the reader to the geometric design process and illustrates how it fits into the larger context of transportation project development. The chapter concludes with a brief discussion of how the road network has developed and evolved in Europe; however, there is only a brief mention of the German road network. The text may benefit, in future editions, from a discussion of how the German road network was developed, the extent of the system, and the design criteria used to develop the system.

Chapter 2 of the text describes the German road classification system. Road classes are divided by the intended function and the entity responsible for the route. The German federal government is responsible for freeways and main roads, both of which are intended to provide a high level of mobility and limited access. Federal states are responsible for state roads, which serve nonlocal traffic within the federal state. District and local community roads serve local traffic (high level of access) and are operated by the districts and local communities, respectively. There are five road category groups considered in the German road design process, designated A–E. Category groups are based on the location (outside or inside urban areas), the surrounding roads (with or without extra lanes), and primary function (connection, site development, or residential) of the road. The chapter concludes with a discussion of how the road categories map to target cruising speeds, the design and operating features of each road category (i.e., expected traffic types, permissible speed, cross section features, at-grade or grade-separation intersection form, and the recommended design speed), and the general design approach for each road category. Readers would likely need access to German design guidelines to fully comprehend the design approaches recommended for the various category groups. The design guidelines are included in the reference list at the end of the text.

In Chapter 3 of the text, the author describes the interaction between the driver, vehicle, and road, based principally on Durth’s closed-loop representation of all three components. The discussion is focused on the driver’s visual capabilities and aptitude to react to various stimuli while driving. Although the material in Chapter 3 is presented independently of the other chapters in the text, it appears that an opportunity exists to connect the material in Chapter 3 to the content in Chapter 4, which contains information on the kinematic equations of motion. These equations are very clearly presented in the first half of Chapter 4, in which the author skillfully shows how the equations are used to develop fundamental braking distance criteria used in road design. Several examples are provided to demonstrate the application of the braking distance formula to readers.

The second part of Chapter 4 describes the resistances that vehicles encounter when in motion and the equations of circular motion. The presentation of material provides the necessary details to understand the derivation of the horizontal curve design formula, and includes a brief discussion concerning the difference between radial and tangential friction forces, which is an important concept not commonly documented in many road design texts. Chapter 4 concludes with a sample calculation to illustrate the vehicle cornering equations in relation to vehicle skidding thresholds.

Chapter 5 defines key speed terminology, which are critical inputs to the German road design process. The design speed is used to determine the geometric elements of a road, similar to current practice in the United States, which is contained in AASHTO’s A Policy on Geometric Design of Highways and Streets. An operating speed (based on the 85th percentile of the free-flow speed distribution) is used to check the German design inputs for new construction and reconstruction projects. There is a documented empirical relationship between the bendiness or radius of curve on an undivided two-lane road and the road width and 85th-percentile speed within the road design process. The author directs the reader to the German Guidelines for Road Facilities for further information concerning the relationship between all of the speed measures and road design policy.

Chapter 6 of the text describes the sight distance criteria that are applied in German road design. Stopping sight distance (SSD) consists of two components: distance traveled during perception and (re)action time and the distance traveled during braking. Key differences between the German SSD policy and that used in the United States are that the perception-(re)action time is 2.0 s in German design guidelines, whereas it is 2.5 s in United States design policy. Additionally, 85th-percentile operating speed is used in German design policy instead of the design speed, which is used in United States policy. The braking distance component of the German SSD criteria uses the tangential road adhesion rather than the rate of driver deceleration, which is used in United States design policy. The German passing sight distance (PSD) criteria include three components: distance covered by the passing vehicle, distance covered by the oncoming vehicle, and a safety margin (assumed to be 25 m). This differs from the United States PSD criteria, which are based on the warrants for marking passing and no-passing zones on two-lane highways in the Manual on Uniform Traffic Control Devices.

Horizontal and vertical alignment design is described in Chapter 7 of the text. Derivations and examples of circular curves and clothoids are covered in detail. The principles of crest and sag vertical curve design are also described in this chapter. Finally, the chapter includes a detailed procedure to check for blind spots when coordinating the horizontal and vertical alignments on two-lane roads. Readers will likely find these checks to have considerable practical value in the rural road design process.

Chapter 8 of the text includes design criteria for the roadway and roadside. Included is guidance related to the number of lanes, lane width, median and shoulder widths, and pedestrian and cyclist lane or path dimensions. Suggested pavement cross-slopes (or camber) are also contained in this chapter, along with recommendations to develop different pavement cross-slopes when traveling from a tangent into a horizontal curve (i.e., superelevation transition or torsion stretch).

Chapter 9 includes basic at-grade intersection and interchange design principles. Basic intersection forms included in the book are three-legged tee intersections, four-legged cross intersections, and roundabouts. Common three-leg interchange forms include trumpet and other forms with direct or semidirect connections. Full and partial cloverleaf interchanges are described as common four-leg interchange forms, whereas other types include the windmill and turbine (four loops of full cloverleaf are replaced with semidirect connections) and the Maltese cross (systems interchange with only direct or semidirect connections).

Chapters 10–12 include discussion related to the road design process, visualization, and emerging research in three-dimensional road design methods, respectively. Chapter 10 provides an excellent narrative on how to develop alignment alternatives for a project and how to assess the quality of the design based on design efficiency, traffic safety, environmental impacts, and cost-effectiveness. A useful introduction to visualization techniques, applications, and related software packages is provided in Chapter 11. Detailed theoretical explanations and examples of several three-dimensional road design assessment methods are provided in Chapter 12.

As indicated in the preface of Fundamentals of Road Design, the text is focused on the theory and application of German design policies and procedures for rural roads. The text can clearly be used as a reference by German road designers. Additionally, the text could be used by international practitioners to learn about how the German road network has evolved from well-established design policy. The text, however, would have limited application for urban road design.

The text is also intended for German university students. The combination of theory and application clearly fulfills this mission. However, the end-of-chapter design problems are limited with respect to computational application of design fundamentals. Rather, the end-of-chapter problems are generally focused on concepts included in each chapter. Because there are differences between German design policy and those in other countries with well-developed road networks, university students may find it interesting to compare the research that has been performed in support of the German design criteria to the research used to develop road design criteria in other countries.