Michiel M. Minderhoud

Extended time-to-collision measures for road traffic safety assessment.

This article describes two new safety indicators based on the time-to-collision notion suitable for comparative road traffic safety analyses. Such safety indicators can be applied in the comparison of a do-nothing case with an adapted situation, e.g. the introduction of intelligent driver support systems. In contrast to the classical time-to-collision value, measured at a cross section, the improved safety indicators use vehicle trajectories collected over a specific time horizon for a certain roadway segment to calculate the overall safety indicator value. Vehicle-specific indicator values as well as safety-critical probabilities can easily be determined from the developed safety measures. Application of the derived safety indicators is demonstrated for the assessment of the potential safety impacts of driver support systems from which it appears that some Autonomous Intelligent Cruise Control (AICC) designs are more safety-critical than the reference case without these systems. It is suggested that the indicator threshold value to be applied in the safety assessment has to be adapted when advanced AICC-systems with safe characteristics are introduced.

ASSESSMENT OF ROADWAY CAPACITY ESTIMATION METHODS

Capacity is a central concept in roadway design and traffic control. Estimation of empirical capacity values in practical circumstances is not a trivial problem; it is very difficult to define capacity in an unambiguous manner. Empirical capacity estimation for uninterrupted roadway sections has been studied. Headways, traffic volumes, speed, and density are traffic data types used to identify four groups of capacity estimation methods. Aspects such as data requirement, location choice, and observation period were investigated for each method. The principles of the different methods and the mathematical derivation of roadway capacity are studied and discussed. Among the methods studied are the headway distribution approaches, the bimodal distribution method, the selected maxima, and the direct probability method. Of the methods based on traffic volume counts, the product limit method is recommended for practical application because of sound underlying theory. An example of the application of this promising method is presented. Attempts to determine the validity of existing roadway capacity estimation methods were disappointing because of the many ambiguities related to the derived capacity values and distributions. A reliable and meaningful estimation of capacity is not yet possible. Lack of a clear definition of the notion of capacity is the main hindrance in understanding what exactly represents the estimated capacity value or distribution in the various methods. If this deficiency is corrected, promising methods for practical use in traffic engineering are the product limit method, the empirical distribution method, and the well-known fundamental diagram method, in that order. The choice of a particular method strongly depends on the available data.

Impact of Intelligent Cruise Control on Motorway Capacity

The introduction of driver support systems such as (autonomous) intelligent cruise control (ICC) is foreseen within a few years. However, it is still uncertain how these systems will affect traffic-flow characteristics on motorways. A simulation study has been conducted to assess the impacts on roadway capacity more precisely. Ten different ICC designs are investigated and compared with a reference situation without such support systems. The equipment penetration rates of the systems studied varied and were 10, 20, 50, to 100 percent. A capacity analysis was performed for a common bottleneck situation: an on-ramp to a two-lane motorway. On the basis of the simulation results, some unexpected findings emerged. Support systems that support the driver at all speeds and that do not restrict the deceleration level give rise to capacity gains of about 12 percent. However, the first-generation ICC systems will hardly increase traffic-flow performance. A special stop-and-go ICC design did not improve the traffic-flow quality. It was found that, regardless of the ICC type, a headway setting of 1.2 s did not change roadway capacity near an on-ramp bottleneck significantly.

The effects of deactivation and (re)activation of ACC on driver behaviour analyzed in real traffic

For the development of a micro-simulation model to estimate the effect of adaptive cruise control (ACC) systems on traffic safety, throughput and environment, data of a field operational test (FOT) was analyzed in which vehicles were equipped with ACC and lane departure warning (LDW) systems. The objective of the research described in this paper was to use this FOT to investigate the effects of deactivation or (re)activation of the ACC on driver behaviour in a real traffic environment. It was found that after the participants deactivated the ACC by pressing the brake pedal, the gap with the lead vehicle was decreased. Resuming the ACC by activating the system or by releasing the throttle after overruling the system resulted in a larger gap between participant and lead vehicle than an overruled ACC or the ACC turned off. The participants overruled the ACC by pressing the throttle mainly to overtake the lead vehicle. By taking these results into account in the micro-simulation environment, a more realistic evaluation of the impact of ACC on safety, throughput and environment could be done.

Motorway Flow Quality Impacts of Advanced Driver Assistance Systems

An ex ante impact assessment of advanced driver assistance systems was conducted with focus on autonomous intelligent cruise control and intelligent speed adaptation. The effects of these systems on efficiency, reliability, driving comfort, and safety were addressed by microsimulation for different penetration levels and bottleneck layouts. The deployment of cruise control improves bottleneck capacity. The bottleneck reliability, however, deteriorates in most cases. No significant changes in traffic safety, expressed by the time drivers are exposed to small time-to-collision values, could be established. From the simulation experiments it appears that intelligent speed adaptation has no effect on capacity and provides no substantial contribution to bottleneck reliability. Contrary to expectations, no significant safety benefits could be established using the study’s assessment approach.

Traffic Flow Impacts of Adaptive Cruise Control Deactivation and (Re)Activation with Cooperative Driver Behavior

In 2006 in the Netherlands, a field operational test was carried out to study the effect of adaptive cruise control (ACC) and lane departure warning on driver behavior and traffic flow in real traffic. To estimate the effect for larger penetration rates, simulations were needed. For a reliable impact assessment, the on- and off-switching of the system should be taken into account. Furthermore, because ACC systems are less cooperative than normal drivers, the reference behavior should include this cooperative behavior. In this research, cooperative driver models for normal drivers were developed as well as an ACC model that included on- and off-switching for the microscopic simulation environment known as the intelligent transportation systems modeler. Parameter settings were validated with data from the pilot. The results clearly show that on- and off-switching behavior has a large influence on traffic flow. This effect depends on the degree of congestion. In congestion, the effect of the ACC is larger. Without the possibility of switching it on and off, the ACC has a positive impact on throughput. However, when the ACC can be switched on and off, it has a negative impact on throughput.

Empirical data on driving behaviour in stop-and-go traffic

In the near future, it is foreseen that car-makers was introduce adaptive cruise control systems extended with a stop-and-go feature, supporting the driver in distance-keeping at low speeds. These intelligent vehicles behave in congested traffic according to driver behaviour rules specified in the controller. Essential part for a correct and efficient functioning of such a congestion assistance system is the specification of the desired time or distance gap to the preceding vehicle. The paper deals with the analysis of empirical gap data, measured during stop-and-go traffic conditions on a Dutch motorway. The data is analyzed and relations between speed and desired gap distance for car and truck drivers have been established. Based on the findings, recommendations are given for the desired gap distance function for intelligent vehicles operating in congested traffic conditions.

Preliminary assessment of the operation of a personal rapid transit system in Eindhoven

Although various personal rapid transit systems (PRT-systems) have been proposed and investigated by different companies and different researchers over previous decades, yet no PRT-system has come to practical operation that is confined to the definition of a PRT. From a literature review it was found that limited attention has been given to the relationship between PRT design variables and performance indicators-such as the (average) waiting time for passengers and the network capacity. The paper investigates, by means of micro-simulation, the feasibility of a circular PRT-system that connects the campus of the Technical University Eindhoven with the main railway station. The features of the applied simulation tool are described, as well as the performance indicators used. For two alternative boarding concepts the planned PRT-system is analyzed. The analysis shows the impact of the different concepts on the quality and efficiency of the system. The conclusion is that there is a trade-off between user quality (expressed as average waiting time at stops) and the system efficiency.

Simulation study after traffic flow quality near toll-lane merging areas

This paper deals with the traffic engineering aspects of the concept of toll-lanes without grade-separated access/egress lanes in metropolitan areas, where motorways - in general - are characterized by a lot of discontinuities. Toll-lanes are projected with the purpose to offer a congestion-free lane for drivers who are willing to pay for the use of such a facility and a high level of service. A simulation study has been performed in order to assess the impacts of advanced driver support systems on the longitudinal and lateral performance near a motorway segment with a toll-lane situated in the median lane and traffic merging to an off-ramp. The conducted simulations varied in the level of toll-lane use, merging proportion, and system penetration in the vehicle fleet. Results from the simulations show the performance benefits of centralized distance control (CDC) for vehicles entering the merging zone. Negative Impacts on throughput were found with the application of an intelligent speed adopter (ISA) with a speed limit of 90 km/h in the merging zone. The combination of the two systems showed small improvements compared with the do-nothing case. The findings show that the application of toll-lanes in combination with advanced driver support systems odd some additional longitudinal and lateral capacity to the main roadway.