By Palmer R. Cook, OD
Recently Ford Motor Company and Domino’s began testing the practicality and customer acceptance of using driverless vehicles to deliver pizzas. Only time will tell if tramping out to the curb on a dark and stormy night to punch a code to lower a car window and harvest your pizza will leap the barriers of safety, practicality and customer acceptance. At least your dinner will not be dependent on the vision of some then nonexistent driver. At best the servo-motors, sensors and computers will work to perfection and tipping for your meal will become history.
Driving is a vision-specific task and will remain so until driverless vehicles are proven by the test of time. Every state has vision screening as part of the licensing process for operating motor vehicles. Unfortunately, a vision screening has limitations, not the least of which is the skill level of the screener.
I recently watched a lady selecting frames for her new lenses. She was having difficulty making a decision, so the optician asked if she would mind if I assisted a bit in the process. She was teetering on the cusp of deciding between a frame with very wide temples, or another style with thinner, high-mounted temples. Both frames suited her facial anatomy. I glanced at her new prescription, and she was in a range of myopia that clearly placed her in the “Glasses To Be Worn For Driving” category.
“I have to wear my glasses while driving,” she informed me, as she showed me her present glasses. The present glasses had temples nearly as wide as the B measurement of the lens and were very similar to one of the new frames she was considering.
She put the old glasses on for me, and I said, “You know in some states you would not be permitted to drive in those.” Her eyes widened a bit, and I continued, “Just look straight at the bridge of my nose, and tell me when my thumb is wiggling.” I held my thumb so that it was to the side and at the level of her pupils and wiggled it.
“Uh,” she said. “My glasses are blocking me, and that’s what happened when I had my driver’s license renewed. I couldn’t pass the side vision test, so the man at the agency told me to take my glasses off, and I passed just fine!”
“Both frames look nice on you,” I replied. “But you might pick the ones with the thinner temples for driving, and perhaps the other one as computer glasses and as spares, since you really shouldn’t be dependent on only one pair of glasses.”
Both the patient and the licensing agency should have been aware of the dangers of blocking peripheral vision. In some situations, peripheral vision can be more important for driving than central vision. Peripheral vision has neither color sensitivity, nor can it be used to discriminate fine detail. But peripheral vision is very good at detecting motion, so it can be effective in perceiving objects such as vehicles that could block your intended lane change and other unexpected hazards.
Most adult patients are licensed to drive, and you have better technology than ever to make driving safer and more comfortable. To use that technology to your patient’s best advantage, you need to be aware of common vision-related problems that drivers are likely to experience.
FATIGUE AND HIGHWAY HYPNOSIS
Physical fatigue shortens reaction time, reduces stereopsis and leads to a reduced blink rate. Fatigue sets in more quickly if vision is blurred, whether by refractive error, the condition of the windshield or glare. Because many drivers drive in a “chin-up” position, they are prone to fatigue that results from prolonged viewing through lens areas with uncorrected marginal astigmatism and other aberrations. PAL wearers may be looking through inappropriate plus power encountered at the upper end of their lens corridors if they drive “chin-up.”
As we approach the holidays, drivers who typically limit themselves to “around-town” driving, may undertake long trips to visit friends or relatives. These drivers may be especially prone to “highway hypnosis” and fatigue, and they should be cautioned about options to help avoid such problems. Highway hypnosis is a loss of conscious attention to the task at hand, and it can be brought on by steadily staring at the road ahead. In this state of altered consciousness, expected responses to routine or repetitive events may be made, but the ability to respond to emergencies or threatening situations may be lacking, or the response time may be much greater. When emerging from highway hypnosis, memory of immediately preceding events may be lacking. This is probably akin to the “boring lecture” syndrome experienced by students upon occasion.
Getting plenty of sleep before a long trip, keeping your eyes moving instead of constantly staring through the same spot on the windshield, adjusting the tilt of your seat occasionally, and maintaining awareness of the effect of stereopsis on objects in your visual field may help you avoid the highway hypnosis. Some drivers have noticed that the view ahead of them sometimes begins to look flat, almost like a picture projected onto their windshield, and this may be symptomatic of the beginning of highway hypnosis.
Windshields are tilted back to improve gas mileage, and a tilted-back windshield is aesthetically pleasing. Unfortunately, this tilt causes light from the dashboard to be reflected into the driver’s eyes. This reflection creates a veiling glare that decreases the contrast of the retinal images. These secondary images are annoying and tiring. By placing a white sheet of paper on your windshield, you can easily see the veiling glare effect of this reflected light. Fortunately, the tilt of most automobile windshields is such that the light is reflected at close to Brewster’s angle so it becomes significantly polarized. As a result, it can be effectively blocked by using polarizing lenses.
For daylight driving, there is no better lens for reducing the dashboard’s veiling glare than polarizing. Every eyecare office should have a polarizing demonstrator to allow patients to better understand the value of polarizing lenses. Because these demonstrators all have polarized lorgnettes of one sort or another, you can invite patients to use the one from your demonstrator to view the effect of reducing the veiling glare in their own cars.
Of course, polarizing does much more than help with the reflected dashboard glare. Reflected light from horizontal surfaces such as ice, snow, water, car roofs, rear decks and even road surfaces can cast annoying, tiring and sometimes hazardous polarized glare. Such glare has been implicated in serious accidents, and wearing polarizing lenses is the one way of limiting these dangerous phenomena.
TINTED LENSES (DAYLIGHT DRIVING)
Tinted lenses can be very helpful for daylight driving, but they should not be used for dawn, dusk, or night driving. Some patients find a light tint helpful for daylight or office-task lighting, but such light tints offer little benefit for daylight driving conditions. At night, a light tint reduces the reach of headlights in direct proportion to the absorption of the tint.
Tinted lenses can be uniform in density from top to bottom, or they can be a gradient design that is darker at the top and lightens as the lines-of-sight pass lower in the lenses. Using a gradient tint offers an option of nearly instant self-adjustment of the lens’ absorption. This can be especially appreciated by patients over 50, who have less active pupils and longer glare recovery times. By lowering their chin, drivers can quickly reduce the retinal illumination as they turn toward the sun or when the conditions vary due to broken overcast conditions.
As all ECPs know, early photochromic lenses did not significantly darken for drivers because there was insufficient UV passing through the windshield. Improvements have been made, and some photochromic products such as Transitions Xtractive are promoted as providing sun protection when exposed to the UV within the passenger compartment.
DriveWear lenses from Younger Optics have a photochromic property that darkens significantly, although they are not as dark as sunglasses lenses. Their transmission properties change as conditions vary. DriveWear’s photochromic actions are nicely balanced to darken appropriately, even with the reduced UV passing through car windows and windshields. They do not over-darken in bright sunlight, so many drivers find them very convenient for shopping and other activities that have drivers in and out of the car to visit well-lighted interior environments.
DriveWear lenses also incorporate a polarizing film to reduce the veiling glare created by windshields as well as polarized glare from other environmental sources. Another advantage of DriveWear for over-50 drivers is that glare recovery time tends to increase with age. When driving across a bridge or in other situations in which the windshield-reflecting glare is intermittently flashing, the glare effect, which would otherwise be exacerbated, is reduced because of the polarized protection. DriveWear lenses are available in both SV and PAL designs.
Serengeti offers polarized Trivex in plano and Rx form. The company says the Rx form, available from +6.00 to –8.00D with cylinders up to 4.00 diopters, is made as a lenticular design to reduce weight and thickness. The lens has a peripheral zone, transitional zone and an optical zone which varies in size according to the Rx. The lens is referred to as the Vario Drive Thin Design, and it can accommodate PALs, lined adds and prism.
TINTED LENSES (REDUCED ILLUMINATION DRIVING)
Generally, the transmission of any lens used for night driving should be 82 percent or higher. While an 82 percent transmission could be safe for night driving, the windshield should be clear, rather than tinted, and the age of the patient and his pupil size under low illumination should be given consideration. It’s estimated that the retinal illumination of a 60-year-old is only about a third of that of a 20-year-old person. This is due to pupillary, corneal, lenticular and media changes within the eye, and it does not take into account changes in the retinal receptors or neural pathways. This strongly suggests that these drivers should have anti-reflective lenses and tint should be avoided for night, dusk and dawn driving.
When fitting high index lenses, especially for patients over 50, it is important to remember that light transmission decreases as index increases. This decrease can be moderated by using AR lenses because they increase the transmission of useful light while decreasing annoying and useless front surface reflections and internally reflected light (Table 1).
In very bright driving situations (e.g., sun on snow, sand or water) and especially in the winter months when the sun is lower in the sky at drive times, sun lenses with 15 percent to 18 percent transmission may be needed. And using sun lenses with a band of dark gradient tint placed high on the lens may be particularly beneficial.
Anti-reflective treatment improves the performance of every lens design, every lens material and every lens prescription. It has advantages for daylight driving because it allows the formation of a clearer, sharper retinal image. For night, dusk and dawn driving AR lenses reduce the effects of “ghost” images caused by light internally reflected within the lenses. In addition, since the light transmission of AR lenses is higher than non-AR lenses of the same material, the reach of headlights will be increased by that same percentage.
NIGHT AND MESOPIC DRIVING
Driving at night or in the reduced illumination of dawn or dusk (mesopic conditions) presents a different set of visual challenges. When the pupils enlarge under reduced illumination, there tends to be a myopic shift. This may be due to accommodative effort stimulated by the darkened environment or the eye’s positive spherical aberration becoming more apparent due to pupillary dilation. Regardless of the cause, this myopic shift has been a subject of discussion for years. Anecdotally, it seems to be a problem primarily affecting those who have not reached their late 40s. This makes sense since at about that age accommodative loss becomes greater, and the pupils become less active and tend to be somewhat miotic.
HEADLIGHTS AND TAILLIGHTS
As many as 7 percent of male drivers and some female drivers may have protan color vision defects which result in a reduced sensitivity to red light. Keeping your taillights clean may make you less likely to be rear-ended, especially in foggy or rainy weather conditions. Clean and clear headlight lenses can extend the reach of your headlights significantly. Non-reflective and non-illuminated objects such as pedestrians or rubble on the roadway may not be visible beyond your headlight pattern on the pavement.
Engineers use the term “minimum sight stopping distance.” It is the sum of the distance traveled during the brake-reaction time, plus the distance required to stop a car. For a reaction time of 2.5 seconds (the standard used in highway design), a car traveling 70 mph will travel a little over 256 feet before any braking can take place. Minimum sight stopping distance can be extended by a windshield free of film and spatter (which can act as secondary distracting light sources) and ultra-clear and clean spectacle lenses. Using AR lenses for night driving that increase light transmission by 8 percent would increase the reach of the headlights by that same percentage and would enhance the driver’s opportunity to react. On the other hand, decreasing light transmission by using a tint or a higher index lens for night driving would tend to delay a driver’s response.
If the plastic headlight lenses that became common over the last decade are not treated or replaced after a few years, they become light diffusers, and transmitted light is scattered. Patients should be aware that plastic headlight lenses which appear to glow instead of remaining clear when the lights are on should be surface-treated for pits and scratches or replaced.
Some doctors, upon hearing complaints about night driving, will check their refractive findings under both high and reduced room illumination. If there is a refractive shift toward less plus or more minus, a discussion with the patient, and perhaps a demo of how distance acuity is affected using trial lenses, is a good idea. In fact, if lenses are to be made for sunglasses, some doctors will place the new prescription in a trial frame and use plano sun lenses in a lens flipper to verify the refractive error with low power minus trial lenses at distance.
The Zeiss DriveSafe lens design (available in SV and PAL form) takes an interesting approach to both patient needs and the challenges of night driving. DriveSafe does not block bright sunlight, but it can be used for daylight driving as well as for office and home lighting settings. First, it has an anti-reflective property which improves the quality of the retinal image for daylight and indoor lighting situations as well as reducing the brightness of “ghost” images under certain conditions including night driving. Second, this coating uses wave cancellation to preferentially cancel more light at the blue end of the spectrum to reduce the blue blur, which would be more out-of-focus when the eye experiences a myopic shift during night driving. The third improvement for night driving could also apply to daylight driving. It is a widening of the corridor and distance zone for PALs. This should reduce head turning to identify potential hazards while driving under any condition.
THE JACK-IN-THE-BOX PHENOMENON OF PLUS LENSES
Patients with moderate to strong plus lenses for distance have a roving-ring scotoma that appears to vary depending on their direction of gaze and head position (Figures 1a and 1b). When driving, peripheral vision carries much of the function of warning us of impending danger. Peripheral vision is sensitive to motion or objects that suddenly appear within our field of view and zone of potential collisions, impacts or other undesirable events. The Jack-in-the-Box phenomenon is especially annoying because when something suddenly appears in the periphery, and the eyes are turned, the object immediately disappears. Looking forward again through the centers of the lenses, the object pops back into view. The only way to get a closer look at the potentially dangerous object is then to make a head turn in its direction.
The Jack-in-the-Box phenomenon cannot be eliminated unless the patient is fitted with contact lenses, but the effect of the scotoma (the wedged area between the purple and copper arrows in Figure 1b) can be reduced by fitting a shorter vertex distance and using a smaller eye size. Aspheric lenses, which would have less BI prism in the lateral periphery, can also narrow this scotoma.
Fatigue while driving long distances tends to cause a slowing of the blink rate, and this can result in excessive evaporation from the tear film. This can be exacerbated by low humidity within the passenger compartment in cold weather. The problem can be further compounded if the vents are turned so that air is blown directly at the driver’s face.
In cold weather, simply using the heater’s “recirculate” setting can help keep the humidity higher in the passenger compartment. Even better, there are inexpensive humidifiers available for use in autos. Dry eye patients may also benefit by wrap frames which restrict airflow across the ocular surfaces. If the patient does prolonged night driving, clear antireflective lenses in a wrap frame can also be helpful.
KNOW YOUR OUTCOMES
When fitting specialty lenses, it’s best to have worn the product in your own prescription, and to encourage your staff to have their own driving lenses. When you have fitted any new lens design, it is a good practice to contact patients after they have had time for adaptation. Patients appreciate that kind of follow-up care, and you will quickly learn about the most appreciated features of the lenses you have recommended.
When making such calls, don’t ask for problems. Patients want to please you, so if you call asking if they are having problems, they are will try to accommodate your request. It’s best to remind them up front that they have a new technology lens product, and that you want to know that their eyes are responding well to it. Reiterate issues and concerns that they had mentioned in your office: Is your night driving easier? Are road signs clearer? Are your eyes less tired and more comfortable after driving?
These are all good, practical questions, and they are couched in relative terms, which make them easier to answer. If the patient is having significant difficulties, those will be brought up and you will have the opportunity to address them. If you are worried about hearing problems or negative issues, you are worrying about the wrong thing. Patients venting to friends, neighbors and co-workers is a more relevant issue. Those kinds of comments are the ones that you should hear so that you can deal with them properly. As you gather feedback about specialty lenses for specific tasks, you will gain a learning curve that will serve you, your patients and your practice well.
Contributing editor Palmer R. Cook, OD, is an optometric educator and optical dispensing expert.