Q: I’m concerned about pedestrians getting killed on the road. I think the lack of use of high beam lights is likely the cause of the drivers not seeing the pedestrians. I have a problem for you to solve. How far do typical low beam lights light the road ahead and how long does it take the average driver to stop their car traveling at 50 mph? Could these lives have been spared had the drivers just used their high beam lights?
A: Having just experienced the darkest day of the year (we always call it the shortest day, but it’s still 24 hours long so that’s not really accurate is it?) it seems appropriate to talk about headlights. With over 15 hours of darkness in every day this week, there’s a good chance that some of your driving will be in the dark.
The law requires that high beam headlights are “of such an intensity as to reveal persons and vehicles at a distance of four hundred fifty feet ahead . . .” Low beams are required to provide 150 feet of visibility. Car manufacturers have to engineer their vehicles to meet the requirements of the law, so let’s accept those distances as minimums for every new car on the road. But unless you’re driving a relatively new car, those distances may not apply to you.
You’ll only achieve the same visibility as the day your car rolled off the assembly line if your lighting system is in the same condition. Over time the brightness of a headlight decreases a bit, headlight alignment can get out of adjustment (which matters a lot more than you might think), and that plastic lens that protects your bulb can get hazy. Just a small amount of misalignment can result in a loss of many feet of visibility, and Research from AAA found that clouded or yellowed lenses produce as little as 20 percent of the amount of light compared to when the lights were new. We mostly think about maintaining our headlights when they burn out, but even what seems like a functioning light might not be providing the visibility it was once capable of.
Just working from the distances required by law, we can get a rough idea of how fast is too fast for your headlights. Keep in mind these numbers are based on dry pavement, straight and level roadway, and an attentive driver. The real world results could easily be worse, but won’t likely be better. The stopping distance for a car traveling 37 mph is 150 feet, so there’s our limit for driving using low beams. At 50 mph, as you asked, stopping distance is 235 feet; clearly way too fast to see a hazard in time while using low beams. At 75 mph you’ll have a stopping distance of 446 feet, barely under the required visibility for high beam headlights.
But you don’t want to drive right at the limit, because the moment circumstances change you’re over it. Road conditions can change rapidly, and overdriving your headlights puts you in a situation with no good exit strategy when a hazard appears.
As to whether pedestrian lives could have been saved by drivers using their high beams, without seeing the details of each crash I just don’t know. But we do know that visibility is a critical aspect of safe driving. Unfortunately, pedestrian fatalities have increased over the past several years. As drivers, we have the responsibility, both legally and morally, to do whatever we can (including maximizing our seeing distance) to avoid a collision with a pedestrian.
I have been using a chart from the textbook Drive Right, chapter 5, unit 5.3 under Stopping Distance. It shows a Reaction Distance (RD) of 50′ and a Braking Distance (BD) of 93′ for 45 mph, equalling 143′ of total stopping distance (TSD). For 55 mph, RD=61′, BD=150′, TSD=211′, and for 65 mph, RD=72′, BD=222′, TSD=294′. A blurb accompanying the chart states “Based on an average reaction time of 3/4 second, with brakes in good condition stopping on dry, level concrete pavement.”
Based on this data, I tell my students that if they’re driving in an area frequented by deer, they’d do well to drive at 45 mph or less to lessen the chance of hitting one. Any thoughts on whether this data is accurate?
A big part of stopping distance comes down to reaction time. In the article above I factored 3/4 of a second to recognize the hazard and another 3/4 of a second to respond to it. I’ve often seen 3/4 of a second used as the total reaction time, and that works for someone who is highly attentive and fully engaged in driving (as we all should be) but I consider it a best case (and not always realistic) scenario. Even an attentive driver can’t see all the possible hazards at the same time. As an example, a lot can happen in front of you while checking your mirrors for a lane change on the freeway. There isn’t one perfect number for reaction time – it varies depending on a lot of factors: driving environment, weather, driver attentiveness, driver age (our reaction time gets longer as we get older). You’re not wrong to use 3/4 of a second as your reaction time; you’re just more optimistic than me.