General introduction and summary

by T.J.T.P. van den Berg

The present report is the result of work performed in fulfilment of a grant in the field of transport safety. In response to the “Call for Proposals with a View to Obtaining Grants in the Field of Transport” (1999/C 41/09) published 16.2.1999, especially devoted to promoting transport safety and sustainable mobility, we proposed in December 2000 a project, combining the earlier research plans of two groups, identified further on as A and B. Both were originally independently developed, both in a European collaborative settings. It was felt that they could benefit from each other’s differences of expertise, one more basic the other more practical with respect to the difficult problem of visual function testing for traffic safety.  In order also to enlarge European collaboration the groups decided to combine to one team. Although group A formally acts as grant holder and project leader, both groups are equally important. In the duration of the projects, both research plans evolved to be stronger intertwined. At the end all researchers of both groups share responsibility for the whole project. The writing of this report though, was the primary work of 2 individuals, Dr. L.J. van Rijn and Dr. G.C. de Wit. This is acknowledged by identification of their names on the respective parts of the report. The original project A was “Entoptic Straylight Test for Traffic Safety” (project leader Van den Berg), and the original project B was “Validity of measurement of visual field and measurement of night vision in the assessment of visual function for granting of driving-licenses(project leader Van Rijn). 

The study reported here, was based on the following consideration. Point 6 of Annex III of directive 91/439/EEG, gives minimum demands for the visual capacity of applicants for driver licensing. These demands should be based on proper knowledge of their relation to traffic safety. Their definition and way of measurement should be unambiguous. Moreover, several visual functions of importance are not included, notably disability glare. It was the intention of the present investigation to further our basis of knowledge with respect to these questions, especially glare, night vision and visual field. Impairments of visual functions display a clear relation to unsafe driving behaviour and to increased rates of traffic convictions. Visual functions display a gradual decline with age, but also younger people sometimes show severe impairments of visual function. It is therefore necessary that demands are being imposed upon the visual functioning of drivers. Parallel to these demands, regular assessment of visual function is mandatory, particularly in older drivers. However, the test procedure itself and the test criteria are subject to considerable discussion. It is not clear as yet which test or combination of tests provide optimal information about visual invalidity. Also, cut-off criteria for each test have not yet been established. Finally, it is not known whether the format of each individual test is most suited for its purpose: the majority of tests that are considered for use in driving-licence assessment procedures, has been developed for clinical purposes, i.e. for the detection and follow-up of ocular pathology. In fact one requires a set of tests that is specifically designed for the detection of functional visual disability. Hence, although most experts acknowledge the fact that driving licence applicants should be tested (at least beyond a certain age), much research must still be performed into the contents of the test procedure and into the various individual tests. It may be anticipated that the necessary knowledge will not be obtained from one single large study, but will rather be gathered from several small studies, each contributing to one or more aspects of the test procedure and of the individual tests. 

The present research covered four aspects of the test procedure.

Chapter 1. Development of a stray light meter for driving-licence application. As yet, glare testing is not an obligatory constituent of the test procedure. However, it is well known that glare can be severely disabling during driving. A major problem in glare testing is the validity of its measurements. The research aimed at the practical application for driver licensing of a measurement technique (the Straylight Meter) that has proven to be very promising under laboratory conditions.

Chapter 2. Clinical studies on visual function testing, especially glare and stray light testing. Visual acuity is by far the most important parameter for the assessment of visual function. It has been argued that night vision parameters (mesopic contrast sensitivity and glare sensitivity) provide valuable information about a drivers’ visual disability. This research aims at investigating to what extent these alternative tests provide additional information about visual disability (particularly cataract) that is not provided by visual acuity. The major outcome is that only straylight testing has added value.

Chapter 3. Methodological considerations of glare and stray light measurement devices in relation to their physical properties. In this chapter several considerations on the assessment of sensitivity of an individual to glare hindrance in traffic are discussed. First the practical importance of glasses as a potential source of straylight and thus glare sensitivity. Second, the relation between alternative ways of glare testing and straylight measurement. Especially the question is discussed in how far the classical instruments for glare testing Mesotest and Nyktotest truly test for glare sensitivity.

Chapter 4. Binocular visual field measurements for driving assessment: comparison of Esterman and Goldmann techniques. Current clinical techniques for measurement of visual field defects aim at the detection of ocular pathology. This has several important drawbacks. A promising alternative is being offered by the Estermann visual field technique. This technique seems very useful for the assessment of visual fields in drivers, because the results are directly related to visual function, not ocular pathology. In the present research project, the relation of the outcome of this alternative test to the outcome of the classic clinical tests will be investigated. The major outcome is that Goldmann and Esterman perform about equally well, but that in view of foreseeable developments a new static automated reference technique is advisable.