Accident Analysis and Prevention: Course Notes 1987/88

This report consists of the notes from a series of lectures given by the authors for a course entitled Accident Analysis and Prevention. The course took place during the second term of a one year Masters degree course in Transport Planning and Engineering run by the Institute for Transport Studies and the Department of Civil Engineering at the University of Leeds. The course consisted of 18 lectures of which 16 are reported on in this document (the remaining two, on Human Factors, are not reported on in this document as no notes were provided). Each lecture represents one chapter of this document, except in two instances where two lectures are covered in one chapter (Chapters 10 and 14). The course first took place in 1988, and at the date of publication has been run for a second time. This report contains the notes for the initial version of the course. A number of changes were made in the content and emphasis of the course during its second run, mainly due to a change of personnel, with different ideas and experiences in the field of accident analysis and prevention. It is likely that each time the course is run, there will be significant changes, but that the notes provided in this document can be considered to contain a number of the core elements of any future version of the course

Problems for Vulnerable Road Users in Great Britain

INTRODUCTION In Britain pedal cycle and, in particular, pedestrian travel are important transport modes for the population. However, given the vulnerable nature of these modes of transport, the number of accidents involving pedestrians and cyclists is high, and in particular the number of killed and seriously injured victims is high. Technical measures to improve safety and efficiency focus almost exclusively on motorized traffic, disregarding the needs of non-motorized traffic participants. In order to determine how technical measures, such as Road Traffic Informatics (RTI) applications, can be used to increase the safety and mobility of pedestrians and cyclists, more information is needed about the causes of accidents to these groups. This report aims to look at a number of the attributes of accidents which involve vulnerable road users and at the characteristics of their travel, in order to identify areas where safety and mobility improvements may be obtained. It is intended to serve as a tool in subsequent stages of this project, and thus is not a general survey of safety and mobility problems for vulnerable road users, but rather a review of those issues that are related to the RTI measures envisaged by the project. The project is aimed at improving VRU safety and mobility both directly, through the enhancement of signalized junctions and pedestrian crossings, and indirectly, through the creation of a model of the traffic system incorporating vulnerable road users. It is intended that this model will permit the routing and guidance of motorized vehicles in such a way as to enhance VRU safety and reduce VRU annoyance and delay from trfic. Both the direct and the indirect measures envisaged will only be relevant to VRU safety and mobility on main roads in urban areas; they are unlikely to be applicable to residential streets or minor roads unless these have substantial VRU flows. The report therefore concentrates (in so far as existing information permits) on VRU safety and mobility on main roads and on VRU use of facilities that are intended to be upgraded through the planned RTI measures. The report is split into two main sections; the first of which will examine safety and mobility problems for vulnerable road users on a national level, and the second will examine safety and mobility problems for vulnerable road users at a more local level, specifically for Bradford in West Yorkshire. Parallel reports are being produced for Sweden and The Netherlands, which will examine the situations regarding the safety and mobility of vulnerable roads users in those countries as a whole, and in one urban area from each (namely the town of Vaxjo in Sweden and the City of Groningen in The Netherlands)

Problems for Vulnerable Road Users in Great Britain, The Netherlands and Sweden

INTRODUCTION In many countries in Europe pedal cycle and pedestrian travel are important transport modes for the population. However, given the vulnerable nature of these modes of transport, the number of accidents involving pedestrians and cyclists is high, and in particular the number of killed and seriously iDjured victims is high. Technical measures to improve safety and efficiency focus almost exclusively on motorized traf£ic, disregarding the needs of non-motorized traffic participants. In order to determine how technical measures, such as Road Trac Informatics (RTI) applications, can be used to increase the safety and mobility of pedestrians and cyclists, more information is needed about the causes of accidents to these groups. The aim of this report is to compare the hdings of three previous reports (Tight, Carsten and Sherborne, 1989; Van Schagen and Rothengatter, 1989; and Ekman and Draskhy, 1989), which examined the problems faced by vulnerable road users (VRUs) in Britain, the Netherlands and Sweden, and in one city from each of those countries, namely Bradford, Groningen and VGo. The aim of these reports was to examine a number of the attributes of accidents which involve VRUs and the charaderistics of their travel, in order to identify areas where safety and mobility improvements may be obtained. It is not intended that this report should provide a general comparison of the safety and mobility problems faced by VRUs in the three countries, but rather a review of those issues that are related to the RTI measures envisaged by the present research programme (DRIVE Project V1031). This project is aimed at improving VRU safety and mobility both directly, through the enhancement of signalized junctions and pedestrian crossings, and indirectly, through the creation of a model of the traffic system incorporating vulnerable road users. It is intended that this model will permit the routing and guidance of motorized vehicles in such a way as to enhance VRU safety and reduce VRU annoyance and delay from traffic. Both the direct and the indirect measures envisaged will only be relevant to VRU safety and mobility on main roads in urban areas; they are unlikely to be applicable to residential streets or minor roads unless these have substantial VRU flows. The report therefore concentrates (in so far as existing information permits) on VRU safety and mobility on main roads and on VRU use of facilities that are intended to be upgraded through the planned RTI measures. This report is split into two main sections, the first of which examines comparisons of safety and mobility at the national level, and the second examines such comparisons at the local (city) level. The analyses undertaken in this report concerning the national level are largely based upon published information, while at the local level, due mainly to the lack of any regularly published information, a number of special tabulations have been made. The information given in the tables is for the most up-to-date year available. As with most international comparisons, this study encountered a number of compatibility problems when trying to bring together data from the three countries involved. These included problems of definition, problems of interpretation and differences in the levels of inaccuracy and underreporting of accident statistics. It is not intended to expand upon the possible effects of such problems at this point, as these have been adequately covered in other reports (see for example Tight et al, 1986). Where possible comparable data have been used in the analyses which follow, however on occasions it was not possible to produce exactly comparable data, and in such cases mention is made of this in the text

A review of factors which influence pedestrian use of the streets: Task 1 report for an EPSRC funded project on measuring pedestrian accessibility

INTRODUCTION This document was written to report the results from Task 1 of the Measuring Pedestrian Accessibility project funded through the EPSRC Future Integrated Transport programme. The project is being carried out by staff at the Institute for Transport Studies (ITS) at the University of Leeds in collaboration with the Pedestrians Association and City of York Council. The overall aim of this project is to identify ways to encourage and enable more people to make more journeys on foot. The specific objectives are to: • quantify attitudes and perceptions held towards walking and the barriers to walking; • study the feasibility of developing a tool that can be used to evaluate pedestrian routes; • undertake validation of the tool. The achievement of these objectives will represent a thorough investigation into the two sides of providing for walking - the physical environment for pedestrians and people's attitudes to walking. Task 1 of the project is an extended literature review and survey of experts to identify an initial list of features that are thought to influence pedestrian use of the streets. The paper that follows is split into a number of sections which look at the different characteristics of pedestrians, factors which affect route choice, factors which affect mode choice, problems faced by pedestrians on our streets and a short review of recent Government (local and national) policy which has influenced pedestrian provision

The development of an automatic method of safety monitoring at Pelican Crossings

This paper reports on the development of a method for automatic monitoring of safety at Pelican crossings. Historically, safety monitoring has typically been carried out using accident data, though given the rarity of such events it is difficult to quickly detect change in accident risk at a particular site. An alternative indicator sometimes used is traffic conflicts, though this data can be time consuming and expensive to collect. The method developed in this paper uses vehicle speeds and decelerations collected using standard in-situ loops and tubes, to determine conflicts using vehicle decelerations and to assess the possibility of automatic safety monitoring at Pelican crossings. Information on signal settings, driver crossing behaviour, pedestrian crossing behaviour and delays, and pedestrian-vehicle conflicts was collected synchronously through a combination of direct observation, video analysis, and analysis of output from tube and loop detectors. Models were developed to predict safety, i.e. pedestrian-vehicle conflicts using vehicle speeds and decelerations

Outlier Detection and Missing Value Estimation in Time Series Traffic Count Data: Final Report of SERC Project GR/G23180.

A serious problem in analysing traffic count data is what to do when missing or extreme values occur, perhaps as a result of a breakdown in automatic counting equipment. The objectives of this current work were to attempt to look at ways of solving this problem by: 1)establishing the applicability of time series and influence function techniques for estimating missing values and detecting outliers in time series traffic data; 2)making a comparative assessment of new techniques with those used by traffic engineers in practice for local, regional or national traffic count systems Two alternative approaches were identified as being potentially useful and these were evaluated and compared with methods currently employed for `cleaning' traffic count series. These were based on evaluating the effect of individual or groups of observations on the estimate of the auto-correlation structure and events influencing a parametric model (ARIMA). These were compared with the existing methods which included visual inspection and smoothing techniques such as the exponentially weighted moving average in which means and variances are updated using observations from the same time and day of week. The results showed advantages and disadvantages for each of the methods. The exponentially weighted moving average method tended to detect unreasonable outliers and also suggested replacements which were consistently larger than could reasonably be expected. Methods based on the autocorrelation structure were reasonably successful in detecting events but the replacement values were suspect particularly when there were groups of values needing replacement. The methods also had problems in the presence of non-stationarity, often detecting outliers which were really a result of the changing level of the data rather than extreme values. In the presence of other events, such as a change in level or seasonality, both the influence function and change in autocorrelation present problems of interpretation since there is no way of distinguishing these events from outliers. It is clear that the outlier problem cannot be separated from that of identifying structural changes as many of the statistics used to identify outliers also respond to structural changes. The ARIMA (1,0,0)(0,1,1)7 was found to describe the vast majority of traffic count series which means that the problem of identifying a starting model can largely be avoided with a high degree of assurance. Unfortunately it is clear that a black-box approach to data validation is prone to error but methods such as those described above lend themselves to an interactive graphics data-validation technique in which outliers and other events are highlighted requiring acceptance or otherwise manually. An adaptive approach to fitting the model may result in something which can be more automatic and this would allow for changes in the underlying model to be accommodated. In conclusion it was found that methods based on the autocorrelation structure are the most computationally efficient but lead to problems of interpretation both between different types of event and in the presence of non-stationarity. Using the residuals from a fitted ARIMA model is the most successful method at finding outliers and distinguishing them from other events, being less expensive than case deletion. The replacement values derived from the ARIMA model were found to be the most accurate

Propensity to Consider Cycling for Commuting Trips

The British Government launched the National Cycling Strategy (NCS) in July 1996. The aims of this strategy are to establish a culture favourable to the increased use of bicycles for all age groups, to develop sound policies and good practice, and to seek out innovative and effective means of fostering accessibility by bicycle. The central target is to double the amount of cycle trips on 1996 figures by 2002; and double it again by 2012. It is hoped that these increases in cycle trips are as a result of people switching their current mode to bicycle. With this increased interest from national and local governments, there is likely to be increasing demand for rigorous evaluation of proposed schemes in terms of increases in levels of cycling, modal shift and, ultimately, the quantified benefits to existing and potential cyclists. Yet, in contrast to the vast amount of research which has been done on enhancing our understanding of the demand for motorised vehicles, relatively little attention has been paid to the slow modes (i.e. walk and cycle). Furthermore, most studies on existing and potential demands for cycling are qualitative rather than quantitative in nature. For instance, attitudinal factors of choosing or not choosing cycling are well documented. However, magnitudes of different factors are seldom calculated. An Economic and Social Research Council (ESRC) sponsored project entitled “Cycling and Urban Mode Choice” (Grant Number: R000237103) started in mid-1997, aiming to identify and quantify the factors that might influence people’s decision to travel to work in urban areas. The aim of this paper is to report the findings of the first stage study of this research project. The first stage study of the “Cycling and Urban Mode Choice” project was to identify and measure the proportion of the general population who would or would not consider cycling to work in urban areas. This study was based on detailed surveys (door-to-door and telephone interviews) of people’s actual mode choices on two what-if situations. The first hypothetical situation is based on provisions of cycle facilities and routes, and the second rests on drastic changes of the current situations of journey to work. Models were built on the survey data to explain people’s choices between driving car, getting a lift, bus, walk and cycle for the journey to work trips

A Simulator Based Evaluation of Speed Reduction Measures for Rural Arterial Roads

In Great Britain accident rates on rural roads are not falling as fast as those on urban roads. In 1993 the number of casualties from accidents on rural A roads was 4% higher than the average for 1981-85, which is the baseline for the Department of Transport target of a one third reduction in road accident casualties by the year 2000. Driving too fast for the conditions is a major factor in accident causation. High speeds in conjunction with the varying geometric conditions common on rural single-carriageway A roads, result in a fatal accident rate which is higher than that for any other type of road. The aim of the research was to investigate, in a safe and controlled manner using the University of Leeds Advanced Driving Simulator, the effectiveness of a variety of measures for reducing driver speeds on rural single-carriageway arterial roads, in order to identify practical and cost-effective combinations of treatments to reduce both the frequency and severity of accidents on such roads. Treatments appropriate to each of three situations were investigated. These were: (1) treatments that reduce speed and speed variance on fairly straight roads (general treatments); (2) treatments that reduce curve entry speeds for sharp bends; (3) treatments that reduce speeds on the approach to and through villages. Treatments investigated included the use of road markings to reduce lane width or produce horizontal deflection; the use of signs both on posts and on the road surface; and the use of optical illusions to affect the driver's perception of speed or road width. Many of the treatments have been used previously, but few have been evaluated in a controlled way. The first phase of the research involved the evaluation of each individual treatment. The treatments were evaluated with respect to their effect on speed, vehicle lateral position, and incidence of overtaking. The second phase of the research involved the evaluation of variations on and combinations of the most effective treatments. Substantial reductions in speeds were obtained by some of the treatments evaluated, for all three situations studied. There were also reductions in speed variance. These reductions are significant both in statistical and practical road safety terms. For the village situation the most effective combination of treatments was the chicane without hatching, yellow or white transverse lines throughout the village, and countdown speed limit signs on the approach to the village. For the bend treatments the most effective treatments were transverse lines with reducing spacing (including a central area filled with transverse lines); a central hatched area; a Wundt illusion (a series of chevrons with increasing angles but constant spacing, pointing towards the driver); and hatched areas at the edges of the road. Further speed reductions may be produced by combining one of the above treatments with the most effective sign treatments — SLOW or a triangular, warning sign style, advisory speed sign painted on the road surface. For the general treatments all those which involved lane narrowing produced speeds significantly different from the control. Shoulders delineated by continuous lines were more effective, than those delineated by broken lines. Shoulder width was not important, but carriageway width was. For central hatching, type of delineation and width of hatched area was not important. The location (central/edge) and type (removing carriageway or lane space) of the narrowing was not important

Cycling & Urban Commuting: Results of Behavioural Mode and Route Choice Models

The research reported here was undertaken as part of an Economic and Social Research Council Project (R000237103) entitled Cycling and Urban Mode Choice. The scope of the study is entirely commuting within an urban context. The objectives of the study were: • to better understand the interactions between car, bus, cycle and walk in an urban context and to explain observed variations in cycle trip rates across towns. • to be able to forecast the effect of a range of improvements to cycling facilities on mode choice • to use the estimated models to evaluate some actual schemes • to provide end users (eg consultants, local authorities, cycling organisations) with a tool to determine the effect of policy measures. In particular, to create models which will enhance the performance of strategic, integrated transport studies. The objectives of this paper are: • to describe the methodology that was used to examine cycling within an urban mode choice context • to outline the collection of data required to estimate behavioural models • to report the results of the estimated mode and route choice models • to provide illustrations of how the model can be used to appraise improvements to cycle facilities and cycling conditions. The structure of this paper is as follows. Section 2 gives a brief overview of the policy context of work on cycling in the UK and previous work in the area. Section 3 describes the methodology used to collect the Revealed Preference (RP) and Stated Preference (SP) data necessary for the model. Section 4 describes the data that was collected. Section 5 gives details of the development of the model and some empirical results from the model that was developed. Section 6 illustrates how the model could be used to appraise improvements in cycle facilities and cycling conditions. Section 7 draws some conclusions from the work

Setar Modelling of Traffic Count Data.

As part of a SERC funded project investigating outlier detection and replacement with transport data, univariate Box-Jenkins (1976) models have already been successfully applied to traffic count series (see Redfern et al, 1992). However, the underlying assumption of normality for ARIMA models implies they are not ideally suited for time series exhibiting certain behavioural characteristics. The limitations of ARIMA models are discussed in some detail by Tong (1983), including problems with time irreversibility, non-normality, cyclicity and asymmetry. Data with irregularly spaced extreme values are unlikely to be modelled well by ARIMA models, which are better suited to data where the probability of a very high value is small. Tong (1983) argues that one way of modelling such non-normal behaviour might be to retain the general ARIMA framework and allow the white noise element to be non-gaussian. As an alternative he proposes abandoning the linearity assumption and defines a group of non linear structures, one of which is the Self-Exciting Threshold Autoregressive (SETAR) model. The model form is described in more detail below but basically consists of two (or more) piecewise linear models, with the time series "tripping" between each model according to its value with respect to a threshold point. The model is called "Self-Exciting" because the indicator variable determining the appropriate linear model for each piece of data is itself a function of the data series. Intuitively this means the mechanism driving the alternation between each model form is not an external input such as a related time series (other models can be defined where this exists), but is actually contained within the series itself. The series is thus Self-Exciting. The three concepts embedded within the SETAR model structure are those of the threshold, limit cycle and time delay, each of which can be illustrated by the diverse applications such models can take. The threshold can be defined as some point beyond which, if the data falls, the series structure changes inherently and so an alternative linear model form would be appropriate. In hydrology this is seen as the non-linearity of soil infiltration, where at the soil saturation point (threshold) a new model for infiltration would become appropriate. Limit cycles describe the stable cyclical phenomena which we sometimes observe within time series. The cyclical behaviour is stationary, ie consists of regular, sustained oscillations and is an intrinsic property of the data. The limit cycle phenomena is physically observable in the field of radio-engineering where a triode valve is used to generate oscillations (see Tong, 1983 for a full description). Essentially the triode value produces self-sustaining oscillations between emitting and collecting electrons, according to the voltage value of a grid placed between the anode and cathode (thereby acting as the threshold indicator). The third essential concept within the SETAR structure is that of the time delay and is perhaps intuitively the easiest to grasp. It can be seen within the field of population biology where many types of non-linear model may apply. For example within the cyclical oscillations of blowfly population data there is an inbuilt "feedback" mechanism given by the hatching period for eggs, which would give rise to a time delay parameter within the model. For some processes this inherent delay may be so small as to be virtually instantaneous and so the delay parameter could be omitted. In general time series Tong (1983) found the SETAR model well suited to the cyclical nature of the Canadian Lynx trapping series and for modelling riverflow systems (Tong, Thanoon & Gudmundsson, 1984). Here we investigate their applicability with time series traffic counts, some of which have exhibited the type of non-linear and cyclical characteristics which could undermine a straightforward linear modelling process