 Good your time of day to you. This video marks the 200th episode of in its many iterations of Plainly Difficult. It's taken 6 years and over 750,000 subscribers to get here. And I'm surprised that you all still keep coming back every week to hear me talk and watch my dodgy cartoons. Well I'm glad you do. Whilst I've got your attention, would you like me to revive one of my early days series from the channel in the future? Such as Strange Places or Interesting People? If so, what would you like me to cover? Let me know in the comments. Anyway that's enough of me talking. Time for the episode of me, well talking. It is a December morning and traffic is flowing along Interstate 75 in McMinn County, Tennessee United States. All is calm until two lorries crash. Both drivers are uninjured and in the face of it, it is a pretty unremarkable event. However, it was set off one of the USA's most deadly pile-ups in living memory. Hello my name is John and welcome to Plainly Difficult. And today's video is on the I-75 Fog Disaster. Background. The I-75 is one hell of a long road, stretching from Florida's Miami Lakes all the way to the Canadian border. Measuring in at 1787 miles, this behemoth highway slices through 6 US states. The first parts of the route had been opened to the public in 1957 and over the following decades sections would be added. The section that will be the focus of today's video is a part of Four Lane Highway, opened in December 1973. More specifically, the SR163 interchange near Calhoun, Tennessee. This section of road is situated between two valleys and is very susceptible to quick-forming fog. In the area, there are several locations in which water collects, such as behind the Chikamuga Dam and in settling ponds of a local paper mill operated by now defunct Bowwater Incorporated. This alarmingly quick and thick fog was responsible for six multi-vehicle collisions in which several people had lost their lives during the 1970s. With a now increasingly notoriously bad stretch of road, the state of Tennessee Department of Transportation decided to try and improve the situation. What they did was to install dual warning signs, which read, extreme dense fog area next five miles, along this stretch in each direction, which contained flashing lights that would activate if fog was detected. In addition, state troopers would be placed along the section of road on foggy days to help reduce the speed of motorists. The initial spate of the section of roads pile-ups came to an end in 1979. This accident would involve 18 vehicles and resulted in three deaths and 14 injuries and luckily all would be quiet for the next 10 years until two trucks would have a minor crash. The disaster. It is 5.30 in the morning on 11th December 1990 and a Tennessee State Highway Trooper is conducting a pretty standard fog check. He found that the morning, as far as he could see, was acceptable on the visibility scale. A light fog would descend on the road, although at the time it was not causing any visibility issues. The fog would continue to intensify as the morning dragged towards 9am. Witnesses would later report that the thickest accumulation was at State Route Overpass 163 near Calhoun. The distance that drivers could see varied between 20 and 10 feet, but some even reported less. Because of the visibility issues, many drivers slowed down, however some did not. This was when at roughly 9.10 in the morning, a freight lorry was struck in the rear by another lorry, which had a tanker trailer. The former was travelling at 25mph and the latter at 45mph. The accident was pretty trivial. Both drivers of the southbound trucks were uninjured. They stopped to inspect the damage, roughly 100 feet south past the overpass. After a few minutes of pleasant trees and checking out the damage, a 1991 model year Oldsmobile saloon vehicle struck the rear of the tanker, which then itself was struck by a truck. This caused the car to crush between the two trucks and caused a fire which killed both driver and passenger in the Oldsmobile. A secondary cluster of crashes occurred a little back north on the southbound lanes, and this involved 72 vehicles. Around the same time on the northbound, traffic started to slow, which were then themselves crashed into. By 9.20am there were two sets of pileups, one in each direction. Some of the collisions had also caused further fires. This resulted in many victims being burned in their vehicles, and even the asphalt road surface being damaged. Some were burnt beyond recognition. Just four minutes after the initial collision, 911 had received its first calls. First responders were quick to arrive on the scene. By 9.30am a triage had been set up for the wounded on the northern side of the highway. The overpass was repurposed as a helicopter landing site, but most seriously injured. Many others were sent via ambulance to local medical centres. More than 200 rescue personnel were involved in rescuing recovery work on the highway. In total, 99 vehicles were involved in the pileups. Sadly, 12 would die with a further 42 injured. The accident site also posed a hazardous materials risk due to the number of freight vehicles involved, and this would require extra time and specialists to clean up. The NTSB attended the scene pretty quickly, to begin investigating the underlying cause of the horrific number of crashes. The investigation. Investigators had quite a large number of witnesses to interview, as nearly 100 vehicles were involved. Because of this, the NTSB would take the best part of two years to release its findings. Initially, the finger of blame was pointed at the bow water paper mill and its settling ponds, but the NTSB couldn't find definitive proof. The striking quickness of the onset of the fog was one of the most startling parts of the story. One eyewitness, who was under the overpass, stated they could not see the bridge, just a few feet above their heads. The NTSB put the root cause of the accidents down to drivers responding to the sudden loss of visibility by operating their vehicles at significantly varying speeds. But even though the NTSB didn't put the blame on bow water for the fog, the US Department of Energy had contradicted this finding nine years before the accident. When it had concluded that the plant was likely to cause three times more fog in the area than that would have naturally occurred. As such, multiple lawsuits were aimed at bow water, which would result in an out-of-court settlement of $10 million with 44 victims and family members of victims in January 1994. But it wasn't all bow waters fault. The state had failed to implement proper fog detection procedures and as such it spent $4.5 million on a computerized fog detection system. This is a pretty sophisticated system with nine forward scatter visibility sensors, 14 microwave radar vehicle detectors and 21 CCTV cameras. All of this is linked to variable speed limits, flashing lights and warning signs. The variable speed limits lowers from 70 to 50 miles an hour at a visibility below one-quarter of a mile and down to 35 miles per hour when the visibility is below 480 feet. Access to the highway in the area is restricted by gates when the visibility drops to below 240 feet. The pile-up highlights the risks of traveling in poor visibility and serves as a sobering reminder that an average commute can become deadly. Well thank you very much for watching this 200th episode of Plainly Difficult. Hopefully we will have well over 200 more. I'm going to have myself a nice vanilla latte to celebrate and I'll see you next week. This is a plain difficult production. All videos on the channel are creative commons attribution share alike license. Plain Difficult videos are produced by me, John, in a currently moderate corner of southern London UK. Help channel grow by liking commenting and subscribing. I'd like to thank my patrons and YouTube members for all your financial support. If you want to follow me further, you can check out my second channel made by John which makes music or you can check out my Twitter. And all that's left to say is Mr Music, play us out please.