The opinions expressed in the studies are those of the consultant and do not necessarily represent the position of the Commission.
An infrastructure that takes into account the functional limitations that accompany ageing can contribute to a reduction of the crash involvement of older people. Taking into account the functional limitations means that the infrastructure provides the driver with enough time to observe, decide and act. This can be achieved by either giving the driver more time, or by making the driving task easier. Examples of infrastructural elements that provide enough time are long acceleration lanes on motorways and large stopping sight distances at intersections. Examples of infrastructural elements that make it easier to observe, decide and act, are an increased letter-height and retro reflectivity of street name signs, and a higher contrast between pavement markings and the carriageway. Based on the problems that older drivers encounter in traffic, infrastructural measures should focus on :
With respect to infrastructural measures that might be able to improve the safety of older adult drivers, it is interesting to know what the older adults themselves indicate to be a problem. Benekohal et al. (1992) posed this question to a group of older adult drivers. Topics mentioned most were:
The same group of researchers also gathered information about the highway features that become important to drivers as they age:
Mesken (2002) posed similar questions to older adult drivers in the Netherlands. Traffic situations in the proximity of intersections that were most often mentioned as being difficult were:
Several studies have revealed that crashes at intersections are overrepresented among crashes involving at-fault drivers of 75 years and older . Intersections can be defined as traffic situations that require complex judgements of speed and distance under pressure of time. Older adults generally have more trouble in meeting these requirements than younger drivers have. Functional limitations that are particularly related to these problems are an increased perception-reaction time and a decline in the ability to discriminate between relevant and irrelevant information. Besides this, a decrease in both visual acuity and mid-peripheral vision plays an important role in safely negotiating an intersection. Mid-peripheral vision is important for being able to see a pedestrian getting ready to cross the street or for being able to see another car approaching the intersection. Infrastructural measures that are important in this respect are:
At the approach of an intersection, the view on other traffic approaching the intersection is largely determined by the angle at which crossing streets meet. The optimal angle is one of 90 degrees. A smaller angle makes it more difficult to get an overview of the intersection and to notice other road users. Road users can compensate for these difficulties by turning their head a little bit more. But since older adult road users generally have restricted head and neck mobility, they will have more trouble with intersections where streets meet at a small angle. Therefore, a right angle junction is important for older adult road users in particular. A secondary benefit of a right angle junction is that it keeps the intersection area as small as possible, thereby reducing the chance of a crash.
Apart from the fact that older adult road users have more trouble overlooking the intersection because of a restricted mobility of head and neck, they also need more time to react (reduced perception-reaction time). A restricted view on the intersection, not only because of a small angle between the intersecting roads, but also as a result of shrubs, trees and buildings blocking the view on the intersection, leaves the driver little time to overlook the intersection and therefore also leaves little time to react. The resulting pressure of time causes more problems for older adult drivers than for younger drivers. Therefore, a restricted sight distance or stopping sight distance has more adverse consequences for older adult drivers than for younger drivers. One can resolve this problem by using a longer perception-reaction time when calculating the sight triangle and the stopping sight distance, with a minimum of 2.5 second.
Older drivers are over-represented in crashes while turning left (see Common crash types among older drivers). These crashes often occur because drivers do not give way to traffic going straight ahead: they either estimate the speed of the approaching vehicle incorrectly, or they simply have not seen it. These causes go together with the various functional limitations that accompany ageing, such as a decrease in depth and motion perception (necessary to determine speed and distance of approaching traffic) and a decline in divided and selective attention (the ability to share attention between various tasks and to separate important from unimportant information respectively). On intersections with traffic lights, errors in giving way and any resulting crashes can be prevented by a conflict free regulation: traffic that can collide does not get a green light simultaneously. In such a case, road users do not have to decide on whether it is safe to turn left.
Intersections without traffic lights should be so designed that road users have an uninterrupted view of the traffic they must cross. Among other things, this means that the two roads should preferably cross at right angles, that bushes and buildings do not obstruct the view, but also that road users do not block each other's view (e.g. cars in the opposite left-turn lane waiting to turn left restrict the driver's view of oncoming traffic in the through lanes).This last situation can be prevented by a positive offset of opposite left-turn lanes. With such an offset, vehicles facing each other do not obstruct each other's view .
Opposite left-turn lanes and the traffic that uses these lanes can restrict the left-turning driver's view of oncoming traffic in the through lanes. The level of blockage depends on how the opposite left-turn lanes are aligned with respect to each other, as well as the type/size of vehicle in the opposing queue. Restricted sight distance can be minimized or eliminated by shifting opposite left-turn lanes to the right (positive offset) so that left-turning drivers do not block each other's view of oncoming traffic. The difference between opposite left-turn lanes that are exactly aligned (no offset) and the situation where the opposite left-turn lane is shifted to the right (positive offset) is shown below:
Source: Staplin et al., 2001
Modern roundabouts seem to have all the required qualities to compensate for the functional limitations of older drivers: left turns are completely eliminated, the driver has fewer decisions to make because of one-way traffic and yield-at-enter, lower speeds allow for more time to decide and act, and view on the intersection is not restricted by small angles between intersecting streets . Therefore, roundabouts could be very relevant for solving the problems the older adult driver encounters when crossing intersections. All the more so, because roundabouts not only reduce the number of crashes, but as a result of lower speeds, also reduce the severity of crashes. This would be especially beneficial to older persons.
However, there are a few drawbacks. First of all, roundabouts are relatively new to motorists and older adults are at a disadvantage in responding to novel, unexpected stimuli . This may even lead to older drivers avoiding roundabouts. Simms  reports avoidance of roundabouts by drivers over 70 years old (even in the United Kingdom); although it is unclear whether the roundabouts that were reported as being avoided were only multi-lane or also single-lane. Mesken  did make a distinction between single-lane and multi-lane roundabouts when asking older adults to indicate what they regarded as difficult traffic situations: whereas 22% of the drivers mentioned multi-lane roundabouts, only 3% mentioned single-lane roundabouts. So older drivers seem to prefer single-lane roundabouts.
In general, right angle connections are more effective in reducing driving speed and provide a better view of the traffic on the roundabout for the drivers that are about to turn onto the roundabout, than tangential connections do . Therefore, especially with the restricted head and neck mobility of the older driver in mind, it is recommended to use right angle connections to the roundabout. Another design element of roundabouts that is related to the view on traffic is the distance between the roundabout and cyclist and/or pedestrian crossings. For a driver leaving the roundabout to be able to have a good view on crossing cyclists and pedestrians that have right-of-way, he/she should be at a right angle with the cyclist/pedestrian crossing. This angle can only be obtained when the crossing is placed at approximately one length of a car from the circulation area (Linderholm, 1996, cited in Brouwer, Herland & Van der Horst )
As people age, visual functions decline and people have more difficulty in dividing attention between, for example, different aspects of the road scene. This makes it more difficult to detect traffic signs and obstacles, and hence to understand the traffic situation at hand. Road design elements can anticipate these difficulties by providing appropriate placement and legibility of traffic signs (e.g. street name signs), conspicuity of obstacles (e.g. kerbs, medians and traffic islands) and recognizable intersection control (who has right-of-way) and lane assignment.
The importance of the legibility of street name signs has to do with the time and effort needed to read the name of the street. One can imagine the kind of danger that could result from a driver who is distracted from the basic driving task for a longer period of time or from a driver who suddenly brakes on approaching a street name sign . Factors that influence the legibility of street name signs are, among others, contrast, luminance, font, letter height, letter width and interletter and interword spacing. These factors become more important as the eyesight of a road user deteriorates. Older adult drivers, because of their age-related deterioration of visual functions, need more contrast, a higher level of background luminance and larger letter sizes than younger drivers to achieve the same level of comprehension. This can be accomplished by raising the requirements for letter size and retroreflectivity of street name signs. The 'Older Driver Highway Design Handbook' recommends a minimum letter-height of 150 mm for use on post-mounted street name signs on all roads where the speed limit exceeds 40 km/h. This letter height may seem high in comparison to European standards. Differences between the North-American and European infrastructure (among which road width) might explain part of these differences, but the fact remains that the older adult will benefit from a higher letter height than is required for the average, younger driver (MUTCD states that the lettering on street name signs should be at least 100 mm high).
Since older adult drivers need more time to act (turning into a street) after having received directional information (e.g. a street name), the placement of street name signs is also very important: older adults should have sufficient time to prepare and execute their actions. Both visibility and prior notification can provide the driver with some extra time to act. Therefore it is recommended to place street name signs post-mounted along the side of the road and to use advance street name signs to improve the visibility of street name signs on major roads and grade-separated junctions. Furthermore, it is recommended to use retroreflective sheeting to provide increased sign conspicuity and legibility. When different street names are used for different directions of travel on a crossroad, the names on intersection street name signs should be separated and accompanied by directional arrows.
The decelerated perception-reaction time of older adult drivers which is responsible for the extra time that older drivers need to act, requires timely warnings of changes in lane configuration. Arrow pavement markings that can provide this kind of information have the disadvantage of being liable to wear, being less visible in bad weather conditions and can be covered by cars at the intersection. Therefore, it is recommended to use overhead lane-use control signs in advance of the intersection as a supplement to pavement markings. Drivers should be able to read these signs at least 5 seconds in advance of the intersection (at operating speed; 50 meters at 36 km/h), regardless of the specific lighting, channelisation or delineation treatments implemented at the intersection .
The decelerated perception-reaction time of older adult drivers that is responsible for the extra time that older drivers need to act, requires that the older driver is informed about the obligatory direction of travel and right-of-way as early as possible. Research in the United States has shown that older adult drivers are overrepresented in wrong-way movements . This overrepresentation can be explained by the older drivers' reduced peripheral vision and their decreased selective attention (the ability to ignore irrelevant information and to discriminate between relevant and irrelevant information). To compensate for these deficits, the most relevant information should be signalled in a dramatic manner to ensure that it receives a high priority for processing in situations where there is a great deal of complexity. This can be accomplished by more conspicuous signs, realized through provision of multiple or advance signs, as well as by placing signs in the driver's field of vision and using signs that are larger in size and have a higher level of retroreflectivity. To prevent wrong-way driving these recommendations are particularly relevant for signs indicating one way roads and 'no entry'. However, the same recommendations apply to traffic signs indicating stop-and yield-controlled intersections, since crashes resulting from failure to yield are also overrepresented among crashes involving at-fault drivers of the age of 75 and older . Moreover, Council & Zegeer (1992, cited in Staplin, Lococo & Byington, ) found that both drivers aged 65-74 years old and drivers aged 75 years and older more frequently failed to yield and more often disregarded the stop-sign than a comparison group did (30-50 years old).
Road markings help the driver to maintain the correct lane position and give him/her a preview of the course of the road ahead. Because of their decreased contrast sensitivity (and their extended perception-reaction time) older drivers need a higher contrast between pavement markings and carriageway to be able to see the markings and have still enough time to act upon them. The same applies to the delineation of discontinuities, such as curbs of traffic islands and medians. The results of several focus group discussions have indicated that older adults have difficulties in seeing these discontinuities, resulting in a possibility of running over them .
Studies in the United States have indicated that driver performance - measured by the probability of exceeding lane limits - was optimized when the perceived brightness contrast between pavement markings and the carriageway was 2.0 . This means that the pavement markings should be at least three times as bright as the carriageway. However, these studies were not specifically focused on older drivers. Another study compared the performance of the top 5 percent of 25-year-olds (the best performing younger drivers) with the bottom 5 percent of 75-year-olds (the worst performing older drivers). Taking the contrast requirements for the latter group into account, Staplin, Lococo & Byington  recommend a minimum in-service contrast level of 3.0 between the painted edge of the carriageway and the road surface for intersections without overhead lighting. For intersections with overhead lighting a minimum in-service contrast level of 2.0 is sufficient.
As far as traffic signals and fixed lighting are concerned, it is important to take the visual limitations of older adults into account. Specifically, the older driver's need for increased levels of luminance and contrast should be weighed against their sensitivity to glare.
Since backplates provide more contrast between the traffic light and its direct surroundings without increasing the risk of blinding, they compromise between lighting, contrast and glare and are a good alternative to increased intensity of light. Backplates should only be omitted when the available space is such that the plate would be too close to the carriageway (CROW, 1996, p. 859; s.n., 1997, Article 1.11). Glare can be further reduced by reducing the intensity of traffics signals during darkness, except when this is unnecessary or undesirable because of the (fixed) lighting of the surroundings .
Lighting is more important for older adults than for the average road user. Both the reduced pupil size and yellowing of the lens of the older adult reduce the amount of light reaching the retina. A consequence of this reduced retinal illumination is that sources must be of higher intensity to be seen at night . Furthermore, timely warnings of unexpected situations and changes in lane configuration and lane width are helpful to older adults because of their increased perception-reaction time and can be achieved by lighting these areas. Therefore, wherever feasible, fixed lighting installations are recommended (a) where the potential for wrong-way movements is indicated through crash experience or engineering judgement; (b) where twilight or nighttime pedestrian volumes are high; and (c) where shifting lane alignment, turn-only lane assignment, or a pavement-width transition forces a path-following adjustment at or near the intersection .
Not only intersections at-grade pose a problem to older adult drivers, but also grade-separated interchanges. A study by Staplin & Lyles  showed that drivers over 75 are overrepresented as the driver at fault in merging and weaving crashes near interchange ramps. Age differences in interchange crashes and violations may be understood in terms of driving task demands and age-related diminished driver capabilities . Merging and weaving on interchanges make high demands on the information processing capacities of the driver. The driver must process a large amount of information in a short period of time and at high speeds, while maintaining or modifying his/her position within the traffic stream. Under these circumstances, several functional limitations of the older adult driver come into play, among which a slower processing of information, reduced visual acuity and peripheral vision, and reduced flexibility of head and neck. When merging and weaving take place in the dark or under bad lighting conditions, poor night vision and increased sensitivity to glare also come into play.
During focus group discussions that were held as part of a study by Lerner & Ratté , older adult drivers indicated that in their opinion interchanges could be improved by:
- eliminating weaving sections and short merge areas; and
- improving exit signing by better graphics and more information on upcoming exits.
Specific measures that apply to these and other road design elements of interchanges relate to:
The usefulness of directional information is determined by its legibility and the time available to act upon the information. Older adult drivers, because of their age-related deterioration of visual functions, need more contrast, a higher level of background luminance and larger letter sizes than younger drivers to achieve the same level of comprehension. This can be accomplished by raising the requirements for letter size and retroreflectivity of direction signs. Research has shown that the American standard for the height of capital letters (the "legibility index" of 50 ft/inch (6.2 metre per 10 mm letter height); also used in e.g. the Netherlands) is insufficient for the visual ability of 30 to 40 percent of drivers who are 65-74 years old, even under favourable contrast conditions . Based on research by Olson  the 'Older Driver Highway Design Handbook' recommends to raise the legibility standard to one that assumes that a 10-mm tall letter is legible at 4 metres.
The time available to act on directional information can be improved by multiple use of advance signing on exits and on lane configuration. Something that is particularly important for the older adult driver, having an increased perception-reaction time.
Merging makes high demands on several visual, information processing, and physical abilities; abilities that decline as people age. In addition, the act of merging has to be performed under pressure of time, since the end of the acceleration lane restricts the possibility to merge. Older adult drivers' difficulties in merging reveal themselves in the extra time needed to merge, at worst possibly ending in a crash. Therefore, the most important measure to be taken to improve the safety of older drivers' merging concerns the length of the acceleration lane. In a survey of drivers aged 65 and older, 49% of those surveyed, reported that the length of motorway entry lanes was a highway feature that was more important to them now than it was 10 years ago . Longer acceleration lanes provide the older driver with extra time to merge and at the same time reduce the pressure of time. There is, however, no research available on the minimal length of acceleration lanes necessary to accommodate older drivers, and general guidelines on acceleration lane lengths differ between countries. For example, the Dutch guidelines prescribe longer lane lengths (350 m) than those recommended in the 'Older Driver Highway Design Handbook' (based on US guidelines; AASHTO ). The fact remains though, that the longer the acceleration lane, the better. In addition, a parallel design (instead of a taper design) should be used for entrance ramps to provide the driver with the ability to obtain full view of following traffic .
As far as deceleration lanes are concerned, it is important to provide a good view on the following curve. The acquired information gives the driver the opportunity to assess the required vehicle control actions (braking, steering). The visibility of the curve can be improved by post-mounted delineators and chevrons .
Lighting is more important for older adults than for the average road user (see fixed lighting). With respect to motorways, fixed lighting should be implemented at exits and entries. The 'Older Driver Highway Design Handbook' recommends complete interchange lighting, but where this is not feasible, a partial interchange lighting system comprised of two high-mast installations per ramp is recommended, with one fixture located on the inner ramp curve near the gore, and one fixture located on the outer curve of the ramp, midway through the controlling curvature. In the case of a partial interchange lighting system, Dutch design guidelines recommend to additionally illuminate a part of the carriageway .
For evidence for the need of highway lighting by older drivers, Staplin et al  refer to the results of a survey by Knoblauch, Nitzburg & Seifert (1997): 70% of the older drivers in the ages of 50 to 97 indicated that more lighting is needed on motorways, especially on interchanges, construction zones and toll plazas.
Measures intended to prevent the most important cause of wrong-way driving among older adult drivers (using the exit as an entry) mainly focus on making the entry more conspicuous: the attention of the driver should be drawn to the entry. Compliance with the existing specifications for the signing and visibility of entries, and maintenance of markings are amongst the most important measures to be taken to prevent wrong-way driving. Furthermore, it has been shown that the "Do not enter/go back" signs used in the Netherlands to warn drivers not to enter the exit, that are located in the median between the exit and entry on half cloverleaf junctions, often can be seen and, therefore, also seem to be intended for drivers on the entry. Due to this false alarm, drivers learn to ignore the signs; which can reduce the effectiveness of signs, or even have an adverse effect. For this reason, it is recommended to place the signs (or shield them) in such a way that they cannot be seen and do not appear to be intended for traffic on the entry. It is also recommended to use wrong-way arrow pavement markings near the terminus on all exit ramps, accompanied by red raised-pavement markers facing wrong-way traffic. Where adjacent entrance and exit ramps intersect with a crossroad, the use of a median separator is recommended, with the nose of the separator delineated with reflectorised paint and extending as close to the crossroad as is practical without obstructing the turning path of vehicles. In addition, it is recommended that the 'obligatory direction of travel' sign is posted on the median separator nose .