In current motorway design practice, microscopic simulation is used to assess the traffic safety and capacity implications of design variants. Many different simulation packages are available, and many researchers have invested effort in improving and calibrating models that describe driving behavior. This paper examines traffic simulation models for merging situations in high traffic volume. Based on a multicriteria analysis, two simulation software packages (VISSIM version 10 and MOTUS) were selected and calibrated using a recently collected set of rich empirical trajectory data from an on-ramp, an off-ramp, and two weaving segments in the Netherlands. The results show that both packages are as yet unable to simulate turbulent traffic around motorway ramps realistically, in terms of lane change locations and headway distribution. This is a drawback when assessing traffic operations for specific designs. The simulated gap acceptance distributions were found to have comparable shapes as they were derived from empirical data. This is an advantage when assessing the traffic safety of a specific design.

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Over the years the characteristics of traffic on Dutch motorways has changed, but its design guidelines did not develop as rapidly and large parts remain unchanged since the first guidelines from the 1970s. During the latest revision of the Dutch motorway design guidelines it became clear that a solid and comprehensive theoretical, or evidence based, background was lacking for the validity of the prescribed ramp spacing and required length for weaving segments. This article presents the underpinning of revising the Dutch design manual for motorways for turbulence in traffic. For this study loop detector data at eight on-ramps and five off-ramps were collected as well as empirical trajectory data at fourteen different on-ramps (three), off-ramps (three) and weaving segments (eight) in The Netherlands. The results show that the areas around ramps that are influenced by turbulence are smaller than described in the design manuals and that, in their present form, the microscopic simulation software packages VISSIM and MOTUS fail to simulate the number and location of lane-changes around ramps realistically.

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In the vicinity of ramps, drivers make route choices, change lanes and in most cases also adjust their speeds. This can trigger anticipatory behaviour by the surrounding vehicles, which are also reflected in lane changes and/or changes in speed. This phenomenon is called turbulence and is widely recognised by the scientific literature and various design guidelines. However the knowledge about the characteristics of turbulence is limited. This study investigates the microscopic characteristics of driving behaviour around 14 different on-ramps (3), off-ramps (3) and weaving segments (8) in The Netherlands, based on unique empirical trajectory data collected from a video camera mounted underneath a hovering helicopter. The data analysis reveals that lane changes caused by merging and diverging vehicles create most turbulence, that an increase in the amount of traffic results in a higher level of turbulence and that an increase in the available length for merging and diverging results in a lower level of turbulence. The results of this study are useful for improving the road design guidelines and for modelling driving behaviour more realistically.

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In the vicinity of motorway ramps, multiple manoeuvres are performed by drivers that are entering or exiting the motorway, and by drivers that anticipate on, or cooperate with, the other entering and exiting vehicles. These manoeuvres involve lane-changes, changes in speed, and changes in headways. This results in changes in lane flow distribution, greater speed variability and changes in headway distribution on the different lanes, with presumably a greater share of small gaps on the outside lane. In literature and motorway design guidelines, this phenomenon is referred to as turbulence. Currently, an explicit definition for turbulence is unavailable. In this thesis, therefore, an explicit definition for turbulence is introduced: “individual changes in speed, headways, and lanes (i.e. lane-changes) in a certain road segment, regardless of the cause of change”. Turbulence is expected to be present in the traffic stream at any given time, and therefore a second definition is introduced: the level of turbulence, which is defined as: “the frequency and intensity of individual changes in speed, headways and lane-changes in a certain road segment, over a certain period of time”...@en

Traffic is more turbulent around motorway ramps due to route choice-related lane changes and anticipatory or cooperative manoeuvres. These manoeuvres result in changes in speed and headways and have a negative influence on traffic safety and capacity. However, the distance upstream where turbulence starts and the distance downstream where it dissolves are yet unknown. In this paper, we propose a new method for detecting the start and end distances of turbulence. This method relies on the analysis of large quantities of empirical loop detector data from multiple on-ramps and off-ramps at different sites. By comparing the traffic operations near ramps to those on a regular motorway section, the length of the turbulence influence area can be estimated. The scope of the research is limited to three-lane motorways in the Netherlands, and shows that the distribution of traffic over the motorway lanes is a useful indicator for turbulence.

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Wat is de impact van zelfrijdende voertuigen op wegontwerp? Witteveen+Bos, Arcadis en TU Delft deden er een verkennend onderzoek naar. Op korte termijn lijkt de impact beperkt, op langere ter-mijn zijn er mogelijkheden voor efficiënter ruimtegebruik en het verhogen van de wegcapaciteit.@en

Several studies in the literature have indicated that interchanges are the most crash-prone areas within the motorway system in number and severity of accidents. The reason is the high level of turbulence as a result of vehicle lane changes and speed variability. To understand the safety consequences of an interchange design (e.g., type of connecting ramps, radii and superelevation of curves, and lane and shoulder widths), an in-depth investigation of driving speed behavior is needed. Such an investigation requires the collection of detailed trajectory data on vehicles on different interchanges. These types of data are rarely available, and as a result, such studies are scarce in the literature. The main objective of this present study was to analyze driver speed behavior on different ramps at interchanges, and to develop an operating speed prediction model as a function of the road design elements. Trajectory data on free-moving vehicles were derived from stabilized video images taken from a camera mounted underneath a helicopter, which hovered over the road areas studied. Data were collected from 29 curves at six freeway-freeway interchanges in the Netherlands. The sample included nine direct connections, 12 semidirect connections, and eight indirect connections. The findings showed that speeds were affected by several road geometric characteristics of the curves, by driver expectancy and design consistency, and by the percentage of trucks in traffic. The operating speed prediction models developed in the study will provide designers with tools to estimate the operating speed during the design process.

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Turbulence in traffic is a commonly known phenomenon, but the exact characteristics of this phenomenon are not yet clear. Turbulence reflects individual changes in speed, headways, and lanes in the traffic stream. The current freeway design guidelines prescribe measures for handling turbulence, such as sufficient ramp spacing and spacing between road discontinuities. In situations where there is little available space between discontinuities, it might be necessary to make a trade-off between costs and safety and operations. For a valid trade-off, more insight is needed on the safety and operations effects of deviations from the guidelines. Much research has been done on the causes of turbulence and their effects on safety and operations. This paper proposes a theoretical framework for the turbulence phenomenon that facilitates the comparison of the available methodologies that can be used to evaluate a freeway design on the matter of turbulence and its impact on traffic operations and safety. The main finding of this review is that the currently available methodologies lack the ability to evaluate the impact of freeway turbulence on operations and safety simultaneously. The paper makes recommendations to overcome the limitations of current methodologies and for further research possibilities to improve these methodologies.

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