Werner Brilon

Useful estimation procedures for critical gaps

Many different methods for the estimation of crtical gaps at unsignalized intersections have been published in the international literature. This paper gives an overview of some of the more important methods. These methods are described by their characteristic properties. For comparison purposes a set of quality criteria has been formulated by which the usefulness of the different methods can be assessed. Among these one aspect seems to be of primary importance. This is the objective that the results of the estimation process should not depend on the traffic volume on the major street during the time of observation. Only if this condition is fulfilled can the estimation be applied under all undersaturated traffic conditions at unsignalized intersections. To test the qualification of some of the estimation methods under this aspect, a series of comprehensive simulations has been performed. As a result, the maximum likelihood procedure and the method developed by Hewitt can be recommended for practical application.

Implementing the Concept of Reliability for Highway Capacity Analysis

A stochastic concept for highway capacity analysis is presented. The capacity of a highway facility is regarded as a random variable instead of a constant value. Thus, the stochastic approach provides new measures of traffic flow performance based on aspects of traffic reliability. A method for the estimation of capacity distribution functions from empirical data based on statistical methods for lifetime data analysis is introduced. This method is derived for the analysis of freeway capacity. However, the stochastic approach also is shown to be applicable to intersections. Results of the analysis of data samples from freeway sections in Germany indicate that freeway capacity is Weibull-distributed with a considerable variance. A Monte Carlo technique based on the stochastic description of capacity is proposed to quantify freeway traffic performance over a whole year. This technique also provides a quantitative assessment for oversaturated conditions.

Implementing the maximum likelihood methodology to measure a driver's critical gap

This paper focuses on the implementation of the maximum likelihood technique to measure critical gap using data collected in the field. A methodology to define gap events is proposed, so that the accepted gaps and the largest rejected gaps could be obtained. The measurement at multi-lane sites and the treatment of major-street right-turn movement are specifically discussed. The paper points out some unusual situations where the proposed method cannot be applied directly. The proposed methodology has been used in NCHRP 3-46 to measure critical gaps under conditions in the U.S.A.

Variability of Speed-Flow Relationships on German Autobahns

On German autobahns the average speed is strongly dependent on traffic volume. In addition to traffic volume, there is a comprehensive set of parameters that also cause a time dependency of speeds on autobahns. Speed-flow relationships play a key role in the determination of the type of highway for a given design flow and a given desired traffic flow quality. The question, however, is whether the speed-flow relationships from the guidelines are reliable over longer periods and to what extent the shortterm speed variations are larger than the flow dependencies. Therefore, the objective of this research project was to determine typical fluctuations of average speeds on autobahns that are not the result of different volumes. Moreover, the consequences of differing speed-flow relationships on the maximum flow (i.e., capacity) should be pointed out. Studies of influences on the speed-flow relationships for German autobahns are presented. In addition to the fundamental influencing factors such as traffic volume...

Capacity at intersections without traffic signals

Two-way stop control is a form of intersection operation in widespread use. Calculations of the level of service are usually based on gap acceptance theory. In the American Highway Capacity Manual, the influence of pedestrians is considered an impedance factor, whereas in the German guideline the influence of pedestrians and bicyclists has not been considered so far. To close this gap, an alternative calculation method, called the conflict technique, has been developed. This method allows consideration of the influence of nonmotorized road users on the traffic performance of motor vehicles. Moreover, the method provides a significant simplification of the entire theoretical approach. Different modalities of operation, such as a zebra crossing at the entries to an intersection, can be considered, as can the fact that some road users do not comply with the priority rules. It is possible to calculate the capacity for one movement from the minor stream from a single equation. To calibrate the calculation method, traffic at several intersections was observed by video and analyzed for traffic volumes, delays, compliance with priority rules, and other parameters. With these field measurements, the calculation method was calibrated to actual road-user behavior. Comparison of the conventional calculation concept based on gap acceptance and the new conflict technique showed that they provide similar results. In particular, the consideration of pedestrians and limited priority effects is a considerable benefit of the new method.

A Comparative Assessment of Stochastic Capacity Estimation Methods

The stochastic nature of highway capacity has gained increasing attention in recent times. For the empirical estimation of capacity distribution functions based on measured traffic data, two methodologies have received considerable application: The direct estimation of breakdown probabilities for groups of traffic volumes on the one hand and the estimation of capacity distribution functions based on statistical models for censored data on the other hand. The objective of the paper is to compare these methods in terms of estimation accuracy, applicability, and consistency of the results. The theoretical differences of both methods as well as the consequences for application are discussed and analyzed based on empirical traffic data as well as data from macroscopic simulation. The analysis yields that the capacity estimation based on models for censored data performs better than the direct breakdown probability estimation technique, particularly concerning the consistency of the estimated capacity distribution functions.

Capacity at Unsignalized Intersections Derived by Conflict Technique

A new simplified theoretical approach for the determination of capacities at unsignalized intersections has been developed on the basis of the method of additive conflict streams. This method is much easier to handle than the method of gap acceptance. It avoids many of the theoretical complications inherent in the method of gap acceptance, which, under certain circumstances, seems to be unrealistic. The new method has been developed for potential intersection configurations when one street has priority over the other. A calibration of the model parameters is given for German conditions. The new procedure can deal with shared lanes, short lanes, flared entries, and cases of so-called limited priority. For the estimation of traffic performance measures such as average delay and queue lengths, the classical methods can be applied.

Recent developments in calculation methods for unsignalized intersections in west Germany

This paper gives a survey of the development of calculation methods for unsignalized intersections in Germany. First of all, some comments and critical notes on the German guideline published in 1972 and the procedures adopted in the HCM 1985 are given. Moreover, the present state of the art, which comprises the calculation of capacities and queues with stationary and partially non-stationary traffic streams by merely mathematical methods and by simulation models, is described. For future practical application in Germany, both a simple calculation method and a practicable simulation model are planned. However, with respect to the correct consideration of impedance effects of traffic streams at intersections, some theoretical questions still have to be discussed up till the introduction of a new guideline.

Two-Lane Rural Highways: The German Experience

A series of investigations was performed in Germany to understand traffic operations on two-lane highways and to make the experiences accessible for traffic performance analysis in practice. As an initial step, traffic flow on several sections of rural two-lane highways was observed. The observations led to a set of empirical speed-flow relationships for highways of different characteristics. On the basis of these results, a microscopic simulation model was calibrated. The model was able to produce a comprehensive set of speed-flow diagrams for all kinds of rural two-lane highways. The diagrams revealed several unexpected properties: a concave shape and nonlinear influences of the external geometric parameters such as curvature or gradients. The simulated results were then approximated by a simple quadratic equation with the parameters depending on geometric conditions and the percentage of heavy vehicles. Thus, the speed-flow diagrams were obtained for the German Highway Capacity Manual. In this case den...

Evaluation of Cellular Automata for Traffic Flow Simulation on Freeway and Urban Streets

A Cellular Automaton is a extremely simplified program for the simulation of complex transportation systems, where the performance velocity is more important than the detailed model accuracy. The first application of the Cellular Automaton for simulation of traffic flows on streets and highways was introduced by Nagel and Schreckenberg [7]. The basic Cellular Automaton model from Nagel-Schreckenberg has been checked against measurements of realistic traffic flow on urban streets and motorways in Germany. It was found that the measured capacities on German motorways cannot be reproduced very well. On urban streets, however, it was very well possible to represent traffic patterns at intersections. The paper describes a completely new concept for the cellular automaton principle to model highway traffic flow. This model uses a time-oriented car-following model. This model accounts for the real driving behavior more precisely than the model from Nagel and Schreckenberg. This paper shows that a Cellular Automaton is generally applicable for simulation of traffic flows. The degree of correspondence with reality depends on the applied car-following model. The new model concept combines realistic modeling with fast computational performance.