Rod Troutbeck

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.

Elderly drivers and simulated visual impairment

Background. The effect of simulated visual impairment on the driving performance of elderly subjects and the relation between changes in driving performance and vision were investigated. Methods. Vision was impaired by goggles simulating the effects of cataracts, binocular visual field restriction, and monocularity. Driving was assessed on a road circuit free of other vehicles. Visual performance was measured using the Humphrey Field Analyser (HFA), the Useful Field of View (UFOV), and the Pelli-Robson chart. Results. The simulated visual impairment significantly decreased driving performance, even though all drivers satisfied the legal visual requirements for driving. Significant correlations between driving performance and the UFOV and Pelli-Robson chart were found.

Effect of visual impairment on driving

The aim of the study was to determine the effect on driving of restricting vision. This was undertaken by comparing the driving performance of young, normal subjects under conditions of simulated visual impairment with a baseline condition. Visual impairment was simulated using goggles designed to replicate the effects of cataracts, binocular visual field restriction, and monocular vision. All subjects had binocular visual acuity greater than 6/12 when wearing the goggles and thus satisfied the visual requirements for a drivers license. Driving performance was assessed on a closed-road circuit for a series of driving tasks including peripheral awareness, maneuvering, reversing, reaction time, speed estimation, road position, and time to complete the course. Simulated cataract resulted in the greatest detriment to driving performance, followed by binocular visual field restriction. The monocular condition did not significantly affect driving performance for any of the driving tasks assessed.

Effect of restriction of the binocular visual field on driving performance.

The importance of the visual field on driving performance was investigated. This was undertaken by simulating binocular visual field defects for a group of young normal subjects and assessing the impact of these defects on performance on a driving course. Constriction of the binocular visual field to 40° or less, significantly increased time taken to complete the course, reduced the ability to detect and correctly identify road signs, avoid obstacles and lo manoeuvre through limited spaces. Accuracy of road positioning and reversing were also impaired. Constriction of the binocular visual field did not significantly affect speed estimation, stopping distance, or the time taken for the reversing and manoeuvring tasks. The monocular condition did not significantly affect performance for any of the driving tasks assessed.

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.

Limited priority merge at unsignalized intersections

The development of a new gap acceptance model based on limited priority for the major-stream is discussed. Field observations were done to identify the merging mechanism taking place at unsignalized intersections under congested conditions. A limited priority system is proposed based on the assumption that the major-stream vehicles could be slightly delayed to accommodate the minor-stream vehicles. Equations for capacity in the limited priority system were presented assuming that the major-stream vehicles have a bunched exponential headway distribution and the minor-stream drivers are both consistent and homogenous. The gap acceptance model based on the limited priority was then applied to the performance of roundabouts. It was found that the limited priority merge can have a significant effect on the entry capacity at two-lane roundabouts. The relationship between the entry capacity and the circulating stream flow at two-lane roudabouts was very close to the straight-line relationship that has been found in the U.K. using an empirical method.

Capacity of Limited-Priority Merge

Unsignalized intersections are the most common intersection type. They can have high cost implications for the community. A new development often has an unsignalized intersection for ingress and egress, and the basis for deciding whether an upgraded intersection is needed is based on the performance of that intersection. The concept of limited priority, in which the major-stream drivers slow and allow minor-stream drivers to enter in front of them if the average delay to the major-stream vehicles is likely to be short, is addressed. Similar behavior could also be caused by minor-stream drivers forcing their way into a priority stream. This behavior leads to a more efficient performance of the merge. Relationships to predict the capacity and the maximum delay to major-stream vehicles when drivers exhibit this limited-priority behavior are developed. The implications of the capacity of this merge type are evaluated.

An exponential relationship for the proportion of free vehicles on arterial roads

The proportion of free vehicles on a road link is dependent on the geometry of the road link and the presence of intersections along the link. This paper details the investigation of the proportion of free vehicles as a function of the link geometry. It is necessary to define the effects of the link geometry before the effect of intersections on the headway distribution can be incorporated. Current methods for describing the proportion of free vehicles on a road link predominantly use linear relationships with the vehicle flow rate. An exponential relationship has been examined and found to be better suited to the modelling of the proportion of free vehicles than the existing linear relationships. In modelling the flow in each lane, the characteristics of the traffic flow were found to be dependent on the lane type being modelled. The kerb lane sustained a higher proportion of free vehicles than the median lane for similar flow rates. This finding is significant in the analysis of priority intersections. The difference is attributed to driver behaviour and is related to the expected ease of overtaking. Using these results the proportion of free vehicles is described as a function of the lane type, lane width and the vehicle flow rate through the use of exponential relationships. The influence of intersections upon the characteristics of the traffic flow can then be incorporated by describing the effect of the proportion of free vehicles on the road link.

Recent Australian Unsignalised Intersection Research and Practices

Australian engineers have been using gap acceptance theory for some time. They understand most of the concepts associated with this theory and are often prepared to modify values in the light of local conditions. The appropriate gap acceptance theory for Australian roundabouts is discussed using the conclusions from observed driver interactions. As a result of these interactions, all circulating streams could be assumed to act as one. The consequences of this assumption are discussed in this paper. The method of estimating the capacity and delays at a roundabout is described here. From the brief discussion of the accident rates at roundabouts it can be confirmed that they are generally safer than other forms of control. A list of current research projects illustrates that unsignalised intersections are still considered to be a major research topic in Australia.

Background for HCM Section on Analysis of Performance of Roundabouts

The background to the Highway Capacity Manual (HCM) section on the analysis of the performance of roundabouts is discussed. The paper has two main objectives: to discuss the background of different techniques used to evaluate the level of service and to describe the method included in the HCM. The paper is in two parts. In the first part, the first objective is addressed and the parameters needed to predict both delay and capacity, which in turn are used to evaluate the level of service, are described. It is concluded that the gap acceptance approach is a reasonable one when the performance of roundabouts is predicted using data from uncongested sites. If there are a significant number of roundabouts with congested approaches, an empirical model should be used. It is also concluded that the results from one country cannot be immediately transferred to another. In the second part of the paper, the recommended practice included in HCM Chapter 10 is given.