Organisation

PhD Defence by Mr Atze Dijkstra

Dept. of Urban and Regional Planning and Geo-information Management

Title of Defence:

En route to safer roads - how road structure and road classification can affect road safety

Summary

The subject of this study is about the influence of network structure and road classification on road safety. Road safety, or unsafety, is usually expressed as the number of crashes or casualties. It is not evident how one can relate what happens at street level to the decisions regarding network design and the elaboration of this design. Traffic circulation can be regarded as the link between these two levels. Behind traffic circulation is the individual who decides to travel from a point of origin to a destination, using a particular route. The route is the starting point for this study. That is because network structure and road classification are important preconditions for traffic circulation and route choice, while the intersecting routes will determine the crash locations. This study will therefore focus on the effects of changing route choices on road safety. The changes in route choice may be the result of:

1. (intended) changes in the structure of the road network
2. a change in traffic circulation, e.g. on account of an alteration of a traffic signal system or of congestion on the main roads
3. instructions to car drivers through navigation systems or route guidance signs

These changes, adaptations and instructions aim to improve the traffic and transport system as a whole. This study is undertaken to find the effect of the changes/instructions enumerated above on the safety of all road users in the road network. This study will show whether an improvement can be attained and how it can be attained.
In this study, an attempt was made to connect different worlds; the world of transport network design, the world of route choice research, the world of micro-simulation modelling, and finally, the world of surrogate safety measures (conflict indicators).

The topic of this study was inspired by the functional requirements of Sustainable Safety, in particular that the shortest and safest route should coincide. Would it be possible to determine whether this requirement can be fulfilled? For answering this question, one needs a method that can represent several safety aspects of trips and routes. Existing methods only show the safety of either intersections or road sections. New methods had to be developed for answering the aforementioned questions. These methods are able to show the results of the improvements for road safety and for traffic flow. The methods can predict the results before the improvements will actually be implemented.
The topic relates to the following five main research areas: road networks, use of the road network, routes and route choice, influencing route choice, and safety aspects of the four previous areas. These main research areas have been subdivided into eight subjects of this study.

1. Characteristics of transportation networks and road networks; influence on both the generation of traffic and the circulation of traffic in the road network
Indicators based on the number of crossing or conflicting vehicles and the type of these conflicts are relevant if road structures need to be selected for increased road safety. Literature shows a number of indications of some road structures having low numbers of crossing vehicles. On the other hand, these structures can have longer distances travelled, which means more exposure to risk. In theory, some structures have low numbers of crossings as well as short distances travelled.

2. Road network structure and road classification: their influence on traffic circulation and its road safety aspects
In the Netherlands, the methods for arriving at a network structure and a road classification are not a foregone conclusion. The design principles and rules of thumb are mostly based on practical experience. The scientific foundation for this set of principles, requirements and rules is very frail.
A series of requirements was formulated in Sustainable Safety with the purpose of improving the functioning of the road network safety and efficiency. In this study, the method of 'integrated network design' was developed to test whether the road network meets these requirements.

3. Route choice in road networks; options to influence route choice
Car drivers mention 'fastest route' and 'shortest route' as main preferences for their route choice. The next preference is 'prior knowledge about a route'. Car drivers hardly ever take 'safety of a route' into consideration when choosing a route. Prior to a trip, most drivers take note of traffic information, mostly by radio, television or Internet. This information will largely determine route choice or departure time. The traffic information en route comes from radio, personal observation, variable message systems, and, for some drivers, a navigation system. A substantial proportion of drivers change their route because of this information.
The expected total effect of navigation systems is: less exposure at the expense of some loss of attention to the other road users.

4. Road safety aspects of road network structure and road classification; results from modelling studies and evaluation studies
Countermeasures regarding network structure and road classification appear to result in crash or casualty reductions ranging from 40 to 80 percent. In most cases, safety on the surrounding roads hardly worsened, because additional measures were taken.

5. Detecting the effects on road safety by changes in route choice; methodological issues and review of different types of studies
From a series of methods, it can be concluded that a few methods are suited for studying road safety effects of changes in the use of the road network, caused by adaptations of the network structure and/or road classification. The effects of these changes on route choice, in particular, determine the choice of method. These methods are: key safety indicators, crash prediction models, comparing features to requirements, and surrogate safety measures (used in micro-simulation models). Only surrogate safety measures will show output for the safety of individual routes.

6. More detailed analysis of road safety indicators; simulated conflicts and recorded crashes
The estimated numbers of conflicts between vehicles in a micro-simulation model (S-Paramics) have been calculated here according to a specified method. The number of conflicts at junctions and the number of passing motor vehicles appear to be quantitatively related to the number of observed crashes for all junctions. The goodness of fit of this relationship could be statistically confirmed.
Models like S-Paramics are, to a great extent, black boxes. For research purposes, the internal modelling of vehicle movements and interactions should be clearer.
The model does not yet take traffic other than motor vehicles into account. Addition of cyclist and pedestrian movements is, however, desired.

7. More detailed analysis of road safety indicators; simulated conflicts, route characteristics and route criteria
The route score consists of a number of safety criteria. Each criterion can be weighted. The route scores change slightly if the weighting of the criteria is adapted. The numbers of conflicts are directly related to the route score. The weightings are not necessary to improve the relation between route score and conflict indicators.
A high value for the route score is related to a lower number of conflicts, to a lower conflict density, and to a lower conflict risk.

8. Changing route choice for more safety; adapting road structure.
The application of an integrated network design method to the study area showed that many connections use roads which have a much higher position in the road network than desired according to the method. This conclusion was supported in other (foreign) studies. This finding clearly shows the structure of the road network in the Netherlands: many main roads (motorways) facilitating as many connections as possible, disregarding the distance between the origin and destination of these connections.

Recommendations

This study shows that, in existing road networks, the safest route does not always coincide with the fastest route. However, car drivers tend to choose the fastest route. Therefore, the fastest routes should be converted to routes that are also the safest.
The importance of road classification for road safety purposes seems to be underestimated. In the Netherlands, the methods for classifying roads could be improved substantially. This study proposes a method for classifying roads: integrated network design. Whether this method really benefits road safety will have to be the subject of further research. This method should be applied to more areas than the present study area. This may show whether the results depend on the (type of) area. From these findings, the issues of generalization and practical application should be resolved.
Road safety will benefit from spreading traffic over a greater number of roads. However, the cost effectiveness of the increase in road length is still a matter to be considered.
Another important issue in this study is the safety indicators (surrogate safety measures). All safety indicators, other than crash figures, are still being researched. A great deal of study is needed before crash figures can be replaced by other safety indicators. However, the conflict indicators in micro-simulation models appear to be a promising approach in finding adequate and accurate safety indicators.
The safety criteria of route choice should be incorporated into navigation systems. A test is recommended to discover the effects of the actual advantages for safety. Full text

Curriculum Vitae

Atze Dijkstra was born in Groningen, on 19 November 1954. He studied civil engineering at the Delft University of Technology. After his graduation, he worked as a researcher at the Transportation and Traffic Engineering Section of the Faculty of Civil Engineering. In 1983 he joined the SWOV Institute for Road Safety Research as a researcher, a post he fulfilled until 1992. From 1992 until 1996 he worked as a researcher and research manager at the OTB Research Institute for Housing, Urban and Mobility Studies (an institute of the Delft University of Technology). His work focused on both goods transport by rail (evaluation of the existing capacity of the rail infrastructure), and on internal goods transport, using the Maasvlakte (Port of Rotterdam) as a case study.
He rejoined the SWOV as a senior researcher in 1996. From 1996 to 1998 he was the project manager of several work packages in European research projects He set up the national course for road safety auditors; a course that has been presented every year since 2001.
From 1999 to 2003 he was the manager of the research programme 'Road design and road safety'.
At the end of 2003 Atze started working on the project 'Route choice in a road network' which ultimately resulted in his thesis. This eight-year project was an opportunity for him to get extensively acquainted with the intricacies of micro-simulation modelling.

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