What Is The 85th Percentile Rule For Speed Limits?






For the most part, drivers tend to take speed limits as a given. They exist, they’re enforced, and there’s not much for drivers to think about other than whether they’re following them. Something some may ponder, though, is how speed limits are actually determined, especially in the case of speed limit signs with rather odd numbers. That comes down, in part, to driving habits; a metric known as the 85th percentile rule takes driver data into account to determine what the limit in a specific area should be.

According to the Department of Transportation, the 85th percentile rule aims to consider how the majority of drivers navigate a roadway. It takes into account the speed at which roughly 85% of drivers on a stretch of road drive, or slightly below. The DOT explains that it collects this data using roadside speed-measuring tools over a 24-hour weekday period, ensuring ample data from a large sample size of drivers. Once that speed is determined, it’s set as the limit to accommodate the majority of reasonable drivers.

With that said, the 85th percentile rule is just one part of how speed limits are determined. There are other methods that authorities use to set speed limits that keep roadways safe for drivers and pedestrians alike.

Other ways speed limits are set

Figuring out an area’s speed limit requires more than knowing the speed at which most drivers navigate a given roadway. The 85th percentile speed is a starting point, but the road’s layout and conditions also influence the speed limit. Pedestrian volume, a road median, and traffic flow, for example, can lead to the limits being lowered or raised.  Digital tools like USLIMITS2 can also help authorities set safe speeds by analyzing average travel speeds, traffic volume, and the roadway itself, among other elements, to determine an ideal speed limit.

Perhaps the most morbid way to set a speed limit is known as the Safe System approach. This glass-half-empty strategy assumes that accidents and general driver error are practically unavoidable. Thus, it looks at the most likely crashes on a given roadway, the individuals most likely to be harmed, and the level of harm they’re likely to endure. It considers how roadway design and speed limits can combine to prioritize traffic flow and human safety.

Though variable speed limits have started appearing across the United States, most roadways will have just one speed limit for drivers to obey. As we’ve shown, these limits aren’t random, but are based on a range of factors, including the 85th-percentile rule.





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Today, when one pictures a “classic Dodge Charger”, the first image that pops up is almost certainly one of the highly desirable Charger models from the late 1960s or early ’70s. Indeed, those early muscle car Chargers are iconic, playing a starring role in the “Dukes of Hazzard” television show and, somewhat more recently, “The Fast and the Furious” films. But as time ticks on, is it time to start appreciating the modern version of the Charger as a potential modern classic?

It’s now been over 20 years since Dodge brought back the Charger nameplate for a spacious four-door sedan with an optional HEMI V8 engine. While the basic Charger R/T was a potent machine for its time, Dodge really took the Charger’s game to the next level for the 2006 model year with the debut of the Charger SRT8. 

The SRT8 model used a larger version of the third-gen HEMI V8 that, combined with other performance upgrades, transformed the sedan into a serious performance car capable of running with its 1960s HEMI ancestors at the drag strip — to say nothing of its vastly superior handling and refinement. In the years that followed, Dodge would continue to improve the Charger’s performance with larger and more powerful HEMI engines, but the significance of the original Charger SRT8 is not to be overlooked.

A muscle car legend reborn for the 2000s

Today, with the modern Charger being such an established part of the car enthusiast world, it’s easy to forget some of the controversy that surrounded its mid-2000s return. Most of it focused on the fact that the beloved muscle car nameplate had been brought back for a four-door sedan rather than a retro-styled coupe. Fortunately, those people looking for that retro coupe would be satisfied by the reborn Dodge Challenger when it arrived a few years later, while the Charger went on to become a highly popular muscle sedan in its own right.

The addition of the SRT8 model to the lineup certainly helped, of course. Under the hood was the larger 6.1-liter HEMI V8, which differed from the standard 5.7-liter HEMI in several ways, not least the displacement. With the 6.1 under the hood, the SRT8 made 425 hp and 420 lb-ft of torque, easily laying down a mid-13-second quarter-mile time in Motor Trend’s hands. This was very quick by mid-2000s standards, especially considering the now-outdated five-speed automatic transmission.

But the SRT8’s performance went beyond just the drag strip. As part of the SRT transformation, Dodge also gave the car larger wheels and tires, a retuned suspension setup, and large Brembo brakes. While this didn’t necessarily make the car an agile road course weapon, it did give the SRT8 an athleticism that belied the Charger’s weight and size. 

The evolution of modern Dodge muscle

What’s even cooler about this era in Chrysler/Dodge performance history is that the Charger was just one of the four-door LX platform cars that the automaker offered with SRT badges and a powerful HEMI engine under the hood. Apart from the Charger, buyers could also choose from the more upscale, but ultimately short-lived SRT version of the Chrysler 300C sedan or the Dodge Magnum SRT8 station wagon.

The original Charger SRT8 marked the beginning of a long run of increasingly powerful, high-performance models. In the early 2010s, the Charger SRT8’s 6.1 HEMI was replaced by the larger and more powerful 6.4/392 HEMI, with that motor eventually becoming available in the less expensive Charger R/T Scat Pack. Then, of course, came the Charger SRT Hellcat, with a 707-hp, supercharged 6.2-liter that turned the car into a genuine super sedan.

So is the original Charger SRT8 a guaranteed future classic? Classified listings show that clean examples still bring decent money today, but the fact that it was followed by improved models may ultimately limit its potential for becoming a true, mega-desirable collector car. Regardless, though, the Charger SRT8’s accomplishments in modern muscle car history are not to be taken lightly.





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