Review of How to Make Two-Lane Rural Roads Safer: Scientific Background and Guide for Practical Application by R. Lamm, A. Beck, T. Ruscher, T. Mailaender, S. Cafiso, and G. La Cava

This book sheds a new light to the design practice of two-lane rural highways, and provides scientific backgrounds and guides that would help highway engineers design safer two-lane rural highways. The guides presented in this book are based on research studies conducted by the authors and others on the relationships between the elements of the horizontal alignment and accident rates. Often, when accidents take place, driver errors are emphasized. However, the authors argue that some elements of the horizontal alignment (the relationship between tangent and curved sections) contribute to increased accident potential, and by eliminating or improving poor combinations of tangents and curves, highway designers can help reduce accident potentials.

The authors found that almost 60% of highway fatalities occur on two-lane rural highways. About half of these fatalities would occur on curved roadway sections. They found that many of these accidents caused by speed errors may be related to inconsistencies in the horizontal alignment design, which would cause the driver to be surprised by sudden changes in the highway’s characteristics, leading him or her to exceed the critical speed of a curve, hence losing control of the vehicle. The authors concluded that curved roadway sections and the associated transition sections would present a great opportunity for reducing accident frequency and severity. Hence, reducing accidents caused by excessive speeds inconsistent with highway conditions or geometry became the core principle of the methods presented in this book.

To unify the analysis of the three consistency criteria (discussed below), the authors defined a curvature change rate of the single curve with transition curves $\left({\mathrm{CCR}}_S\right)$ to be used as a parameter to evaluate the safety of transition from a tangent segment to a curve segment of a highway. The accident rate used in this analysis relates the number of accidents that occur on an investigated section in a given time period to vehicle-kilometers traveled on the section. They found that there was a statistically significant correlation between horizontal alignment design (expressed by ${\mathrm{CCR}}_S$ ) and safety (expressed by the accident rate). Using these two factors, the authors classified the horizontal alignment design into three design classes: good, fair, and poor design.

In Chapters 1 and 2, the authors present three quantitative safety criteria which, when properly applied, are intended to provide rural two-lane highways with design consistency, operating speed consistency, and driving dynamic consistency and help improve the highway designer’s ability to analyze two-lane rural roads, thus providing safer designs. These criteria are summarized below for the prospective readers. These are the main concepts that the authors proposed in this book to improve two-lane highway geometric design.

Safety Criterion I states that the difference between design speed $V_d$ and the actual operating speed $V85$ should be within certain defined values, expressed as $\mid V{85}_i-V_d\mid$ . This means that the design speed $V_d$ should remain constant on longer roadway sections, and simultaneously, should match actual driving behavior, expressed by the 85th percentile speed $\left(V85\right)$ . The authors defined three speed-difference classes that define good, fair, and poor designs. They also provide three corresponding ranges in terms of $\mid {\mathrm{CCR}}_{Si}-\varnothing {\mathrm{CCR}}_S\mid$ , where $i$ corresponds to the element number under consideration, and $\varnothing {\mathrm{CCR}}_S$ is the average ${\mathrm{CCR}}_S$ for the observed highway section without regarding tangents.

Safety Criterion II is based on a concept that a well balanced operating speed sequence between successive design elements (curve sites and/or independent tangents) promotes a consistent, economic pattern. This criterion evaluates the speed difference $\mid V{85}_i-V{85}_{i+1}\mid$ between the 85th percentile speeds of successive design elements. Similar to Safety Criterion II, they provide three speed-difference classes that define good, fair, and poor designs. They also provide three corresponding ranges in terms of $\mid {\mathrm{CCR}}_{Si}-{\mathrm{CCR}}_{Si+1}\mid$ , where $i$ corresponds to the element number under consideration, and $i+1$ the element number next to the element under consideration.

Safety Criterion III incorporates driving dynamic aspects, especially when driving through curves. Safety Criteria I and II do not cover the dynamic aspects of driving through curves. Safety Criterion III compares side friction assumed $\left(f_{RA}\right)$ for curve design with side friction demanded $\left(f_{RD}\right)$ for cars riding through the curve at the 85th percentile speed level. Similar to Safety Criteria I and II, three corresponding ranges in terms of ${\mathrm{CCR}}_{Si}$ are provided, where $i$ corresponds to the element number under consideration.

These three safety criteria are combined in an overall safety module. In the safety module concept, good design section receives a weighting factor of $+1.0$ ; fair, 0.0; and poor, $-1.0$ . By using this concept, the safety module helps highway designers identify the design class of an element he or she is working on. The authors present a flowchart of alignment design with respect to safety. It is a clear guideline to incorporate safety in alignment design. When the proposed method is used, every design element of a two-lane highway can be classified by the overall safety module.

In Chapter 3, the authors present the results of two studies that compared the actual accident situation with the results of the safety criteria. The results of the two studies indicated that there were good correlations between the three safety criteria and accident occurrences.

The authors present four case studies in Chapter 4 to prove the point, applying the safety criteria and safety module. These case studies are very helpful to the reader to understand the concept and its applications. The authors explain step-by-step how the three safety criteria and the safety module are applied to each element in a highway section considered for analysis.

Chapter 5 discusses the level of influence road equipment has (traffic control devices in the United States) on traffic safety. Three levels of road equipment on traffic safety were considered. Level-1 road equipment includes road markings such as edge line marking, solid centerline, and broken centerline, etc. Level-2 road equipment includes traffic control devices such as curve warning signs, crest warning sign, speed limit signs, chevron alignment signs with up to three arrows, as well as combinations of these. Level-3 road equipment includes traffic control devices such as multiple chevron alignment signs with more than three arrows as well as combinations with Level-2 road equipment. The authors provide data showing the relationship between the three design classes (good, fair, poor design) as expressed by ${\mathrm{CCR}}_S$ and the accident rate and the accident cost rate. This chapter appears somewhat less complete compared to the previous four chapters; however, it shows that regardless the level of road equipment, the design quality as indicated by ${\mathrm{CCR}}_S$ values is an indicator of the level of safety of two-lane rural highways.

The authors conclude the book by stating that the three safety criteria presented in the book “can strongly support the work of the highway engineer to reduce accident risk and severity.”

The concept provided by the authors is simple to understand and apply. Compared to the discussions of highway alignment design (especially those of horizontal and vertical alignment designs) contained in AASHTO’s Policy on Geometric Design of Highways and Streets, this book provides a straight forward yet scientifically supported approach to the alignment design of two-lane rural highways.

The reader will benefit from the new design guides presented in this book, which will help highway designers improve the design of two-lane rural alignment design and reduce future potential accidents. I recommend this book to instructors who teach highway geometric design and practitioners who are involved in the design of two-lane rural highways. The book will give the reader a fresh view at highway geometric design.