Background and Project Objective

The “Guidelines for the Construction of Rural Roads (RAL)” (FGSV 2012) distinguish between four design classes (EKL) with regard to design standards. Rural roads in EKL 1 and EKL 2 serve important supraregional and regional connecting functions within the German road network, with correspondingly high requirements for travel speeds; therefore, they should be constructed with three lanes throughout in EKL 1 and in sections in EKL 2 . While three-lane rural roads throughout their entire length, in combination with grade-separated interchanges—similar to highways—had already been extensively studied, relevant findings were lacking for rural roads with three lanes in certain sections, in combination with junctions and intersections equipped with traffic signals. 

In cases of high traffic volumes on short sections of the rural road network, the RAL in both EKLs allow for the use of a narrow, two-lane, four-lane cross-section without having to switch to the highway regulations. However, compared to the “Guidelines for the Construction of Highways (RAA)” (FGSV 2008), there are uncertainties regarding the limits of application for the narrower highway cross-sections with hard shoulders.

The goal of the project is to update the application guidelines for 

1. two-lane cross-sections with and without hard shoulders, as well as 
2. sequences of interchanges 

on EKL 1 and EKL 2 rural roads. The focus is on how cross-section design and interchange layout affect actual driving behavior, traffic flow, and traffic safety, and what consequences this has for future planning and design specifications.

Approach

The project specifically examines:

• driving behavior on rural roads, 
• the influence of intersections on driving behavior along the route,
• the impact of the number of lanes on speeds and traffic flow,
• the quality of traffic flow throughout the year,
• traffic safety at the studied road sections and junctions.

A key methodological component is the use of Floating Car Data (FCD). This consists of anonymized GPS data collected from vehicles traveling in regular traffic. It enables comprehensive conclusions about actual driving behavior without the need for separate measurements on every road segment. Based on this data, speed distributions are determined, and the influence of intersections and the number of lanes is analyzed.

In addition, a year-round analysis of traffic flow is conducted. Rather than considering only individual reference dates or typical weekdays, traffic is evaluated over the course of an entire year. This allows for a realistic depiction of seasonal fluctuations, peak traffic volumes, and special traffic situations.

The study of traffic flow is combined with an analysis of traffic safety. In addition, a simulator study is incorporated, in which driving behavior is examined under controlled conditions. This also allows for the consideration of situations and design variations that do not occur—or occur only rarely—in the real road network.

The unique aspect of the project lies in the integration of three perspectives: traffic safety, traffic flow, and driving behavior from the simulator study. These are not evaluated separately but rather in conjunction with one another. This creates a holistic foundation from which well-founded and practical recommendations can be derived for the application of multi-lane cross-sections and for the design of a series of intersections on rural roads.

Project partners:

• Prof. Dr. Evelyn Ferstl, Dr. Rul von Stülpnagel (Department of Cognitive Science, University of Freiburg)
• Dr.-Ing. Matthias Zimmermann (Department of Road Design and Operations, Karlsruhe Institute of Technology, KIT)
• Thomas Klemmer, M.S. in Medical Systems Science, Infralytics GmbH