I always notice construction zones, especially on major highways or interstates, will have a speed limit that drops from 70 to 55 or similar, but 55 doesn’t seem to be a particularly safer speed for the workers. Is it just psychology?
I always notice construction zones, especially on major highways or interstates, will have a speed limit that drops from 70 to 55 or similar, but 55 doesn’t seem to be a particularly safer speed for the workers. Is it just psychology?
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Cutting the speed too much makes it a hazard for rear end collisions. It’s a balance, 55 is still safer than 70.
Kinetic energy increases as speed squared.
If your kinetic energy is K at 55 miles an hour, it’s not 20% higher at 70; it’s 66% higher. That’s a lot, especially if you have to brake or steer.
You are quite correct that getting smacked at 55MPH is about as likely to clock you out of your shift indefinitely as 70MPH [ETA: it’s apparently 75% likely fatal to 100% likely fatal; I like neither of those numbers next to “fatal”, but I amend my previous statement that “about as likely” is not accurate]. But it makes a huge difference in drivers being able to see and react to you to avoid collision in the first place.
They want you to slow down and pay attention. It’s likely less about the speed it’s about the attention.
Going slower gives you more time to react to things. So you are less likely to hit a worker at the slower speed.
Risk of dealth is about 75% at 55 MPH, vs close to 100% at 70 MPH. See https://nacto.org/wp-content/uploads/2011PedestrianRiskVsSpeed.pdf. Also, before hiting someone/thing, you usually have some amount of time to slam on your brakes first and slow down. So you might have time to go from 55 to 30 or from 70 to 45. And there is a huge difference in outcomes between those two speeds (same source).
Reducing the speed allows for more time to observe for hazards (which has increased when there is roadworks), better accuracy for manoeuvres (for example, if lanes have been narrowed or twist/turn), and reduces the energy in any collisions (so more likely for people and items to survive collisions).
Your perception distance ability to stop quickly are significantly lower the faster you’re going. For instance, at 50mph, it would take you somewhere around 200 feet between the time you see the thing to stop for and when you stop moving, versus about 400 feet at 70mph. If you hit a worker at either, it’s not going to be good, but if you have the extra time and space to avoid them, that would be ideal.
It’s been a while since 55 was the normal speed limit on highways and interstates, so maybe I’m just misremembering this, but I feel like construction speed zones then only reduced the speed limit to 50 or 45. 45mph is still extremely dangerous for the workers. They’re just as unlikely to survive getting hit at 45 as they are at 55. It pretty much comes down to reducing speed should result in increased reaction times for drivers.
Cars are also built incredibly better today than they were 30ish years ago when speed limits over 55 were rare and practically felt like you were flying. Sensors, automatic breaking, crumple zones, etc, all contribute to less fatalities and serious injuries in accidents. The workers themselves will always be at risk when they’re on the ground, but the damage sustained from a vehicle when it strikes a piece of heavy equipment will be less along with the vehicle occupants having a higher chance for survival.
It also comes down to statistics. On average, about 100 DOT workers across the US are killed in construction zone accidents. I’m not trying to minimize the loss of 100 people, but when you think about how many workers there are across the country, that number has to be in the 10s of thousands, if not more. So 100 people dying is barely a fraction of a percent. That also means that when you consider the amount of vehicles that safely pass through these construction zones each year, over 99% of them are doing it safely.
Reducing the speed limit in work zones would also lead to more congestion, more road rage, more accidents, and more delays in the transportation of things. Unless more DOT workers are being struck by vehicles on the side of the road, there’s really no need to reduce the speed limit below 45.
Since you haven’t specified units, I will assume you’re talking about MegaMiles per MegaHour (MmpMhr)
At 50 MmpMhr, total stopping distance is about 53 metres. At 70 MmpMhr, it’s 96 metres. Thats almost double, which is hugely significant. (Based on dry conditions, average UK car).
That is thinking distance + actual braking distance, with what I assume is assumed to be an attentive driver. Knowing some of the drivers I see on the roads, it’s probably a tad more.
Source: https://www.theaa.com/breakdown-cover/advice/stopping-distances
Because studies have shown that the frequency and lethality of traffic accidents is directly related to the speed that vehicles travel at. I.e. the faster a vehicle moves, the more likely it is to be in an accident and the more likely that accident involves a death of some kind.
A reasonable explanation for this is that drivers have more time to react when they drive at slower speeds, which allows them to avoid accidents, or to take action to make the accident less severe (i.e. they have more time to brake).
You are right that 70 and 55 is both lethal speeds. But stopping distance is squared of the speed. If you spot something while going 70 you first need to slow down to 55 before slowing down to a stop. And while slowing down from 70 to 55 you will be going at those high speeds so you cross a lot of distance. So going 55 means you can stop if you see something ahead while at 70 you will crash into it at high speeds.
Adding to this the kinetic energy is also squared with speed. So a barrier might stop you at 55 but have no chance at stopping you at 70. In construction zones the lanes also tend to be narrower. You can not keep the car straight in the lane but will have to steer from left to right according to what the car is doing. At 70 you are going faster so the car will move left and right in the lane more. Slowing down to 55 means you can keep to a narrower lane.
Overall slowing down from 70 to 55 have a huge impact on the safety of both yourself and the workers in the construction zone. It may be a small decrease in speed and have very little impact on traveling time but it have a huge impact on safety.
Wild. In Germany, the nation with unlimited speed on a lot of highways, construction zone speed is 60km/h or about 35mph.
Reduced speed allows for more reaction time on the driver and worker. Barriers work better at reduced speed. Injuries are less severe at reduced speeds.
Administrative laziness. Way over 90% of the time, an active construction zone has no actual workers present. Because the construction zone is activated when there is no need, it cannot restrict the speed all that much or it would be completely ignored by traffic. Over use of safety restrictions is what makes them ineffective.
Time – you have more time to react at a lower speed.
Survivability – construction equipment is usually heavier and sturdier than your car is, so you have better odds of surviving an impact with trucks or equipment in the construction area at a lower speed. Similarly, a construction worker has better odds of survival if they do get hit by a car at a lower speed.
Road surface – often the construction may result in the road being covered in gravel, dirt, water, mud, or metal plates. Also the road surface itself may be damaged, scored, or uneven. This means potentially having less traction than normal as well as bumps or sudden changes in surface height which could be dangerous if traversed at high speeds. That’s one reason why the signs stay up and the reduced speed is still enforced even when no work is actually being done.
Trucks – things have to be transported in and out of the construction site, often using the road in the process. It is easier and safer for trucks to enter and leave construction sites when traffic is moving slower.
Wind – every vehicle produces a wave of air pressure when it’s moving. This wave pushes against things to the side of the vehicle as it passes by and this can present a hazard to the workers. This effect is less powerful at lower speeds.
Edit: nails & sharp objects – the construction may result in nails or other sharp objects on the road which can puncture tires. It is much easier and safer to control a vehicle with a flat tire at lower speeds.
OP please watch this video for a better understanding of how speed impacts your inertial momentum.
Calculating a Car Crash – Numberphile
It may not seem like much difference cutting it from 65-70 down to 45-55, but that’s enough to improve reaction times, increase the effectiveness of barriers, and decrease the amount of kinetic energy that car can deliver into anything it impacts.
The concern is construction zones is that typically the road will narrow. The worry isn’t that workers may get hit, but rather that a driver will lose control, hit the barrier, and then cause havoc on the construction site. Reducing the speed from 65 to 45 makes this event far less likely on the narrow road.
If you hit someone driving 55 or 70, they have similarly low chances.
But in case of emergency you can break from 55 to 25 and from 70 to 60 on the same distance. That starts to make a lot of difference…
Safety and speed limits in the US are mostly just window dressing. They’re there to aid the prosecution when/if that becomes necessary – definitely not to prevent speeding or enhance safety.
There is a direct relationship between speed and risk. In almost no cases will slowing down even a tiny bit have zero effect on risk. Even 1mph slower will (with enough cars and enough time to overcome statistical noise) result in fewer crashes, fewer injuries, and fewer deaths.