Nothing lasts forever.

In today's world, the average lifespan of a car is approximately 200,000 miles. At around this milestone, most vehicles will hit what many in the industry call a point of "planned obsolescence", in which general wear and tear eventually renders a car unusable.

And when you stop to think about it, running backs in the NFL are a lot like cars.

If every collision in the NFL is "like a car crash," as some players have described it, then there's only so many carries and hits a player can take before they begin to break down and hit that point of "planned obsolescence."

But exactly how much is too much? At what point do the touches finally take their toll and push players into the twilight of their careers?

The 1,800-Carry Cliff

To answer this question I first collected data on every NFL running back whose entire career fell within the past 15 seasons, all while accumulating at least 2,000 carries.

This produced a list of the following eight tailbacks:
LaDainian Tomlinson (3,174 carries), Steven Jackson (2,743 carries), Thomas Jones (2,678 carries), Jamal Lewis (2,542 carries), Frank Gore (2,442 carries), Clinton Portis (2,230 carries), Shaun Alexander (2,187 carries), and Willis McGahee (2,095 carries).

To find the point at which a player's accumulated workload begins to seriously hamper on-field performance, I then plotted the average career carries for this group of backs against their average rushing yards, touchdowns, and Net Expected Points (NEP) for each season of their careers. Rather than go into detail on what exactly NEP is, I'll let you read about it in the glossary, but in short, NEP is our in-house metric to measure a player's production above or below expectation.

What this revealed was a very interesting and readily apparent trend when players reached the 1,800-carry milestone.

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What we find is that, when players hit this mark, rushing yards take a sharp decline the following season, and only continues to plummet in every season thereafter. The same occurs when we look at rushing touchdowns.

And a look at Rushing NEP (normalized to each running back's best season) reveals the exact same trend as well.

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Upon reaching 1,800 carries, Rushing NEP takes an even steeper fall the following season than both rushing yards and touchdowns.

Indeed, when we take the average rushing output from these players in the season where they hit the 1,800 carry mark and compare these numbers to their averages the subsequent season, we find a precipitous drop in every major rushing category.

In the time span encompassing these running backs' 1,800 carry seasons and their next ones, the average rushing yards and touchdowns for these players fall by more than 18% and 27%, respectively. Even more impressively, Rushing NEP falls by nearly 800% over this same time span. Yes, you read that correctly, 800%.

Thus, it becomes obvious that this drop in production for these players after hitting the 1,800 carry mark not only has severe on-field consequences for their respective teams, but also marks their inevitable decline out of the league.

Shaun Alexander and the Lemon Law

When it comes to falling off the 1,800-carry cliff, no player exemplifies this better than former Seattle Seahawks running back Shaun Alexander.

Following a performance for the ages in which Alexander led the league in rushing yards and touchdowns to propel the Seahawks to their first ever Super Bowl berth -- a feat that would garner him league MVP honors that season -- Seattle re-signed Alexander to an eight-year, $62 million contract.

Just a little under $8 million a year to lock down the services of last season's best tailback -- the same tailback whose rushing numbers have improved in nearly every single season he's been in the league? Pretty good deal, right? The Seahawks front office sure thought so.

There was only one problem.

Alexander's prolific 2005 season that helped him earn this gigantic contract also happened to come right before he crossed the 1,800 carry mark. Uh-oh.

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Injuries and a perceived unwillingness to take contact would facilitate Alexander's rapid fall from grace. Following the season where he hit 1,800 carries, Alexander's rushing yards dropped from 1,880 yards in 2005 to 896 yards in 2006 -- a 52% drop. Even worse, his touchdowns dropped from 27 in 2005 to just 7 in 2006 -- a 74% drop in production.

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But worst of all was the Alexander's drop in Rushing NEP. In just one season Shaun's Rushing NEP dropped a total of 87.9 points, from 66.11 in 2005 to -21.76 the following year.

Just three seasons after signing his eight-year, $62 million contract, Alexander found himself out of a job and out of the league.

While few could blame the Seahawks front office for signing Alexander to such a large contract at the time -- one that would eventually cost them All-Pro offensive guard Steve Hutchinson -- the data suggests perhaps Alexander's fall could have been predicted.

Faced with a similar situation this offseason, the Cowboys decided to bet on the strength of their offensive line and let last season's rushing leader, DeMarco Murray, leave for the Philadelphia Eagles. One can't help but wonder if the lessons learned from the Seahawks decision to pay Shaun Alexander over Steve Hutchinson had at least some influence on Dallas' decision not to re-sign their star running back.

Buyer Beware: The Breakdown Candidates

Knowing what we know now about the dangers of the 1,800 carry mark, which players stand at the edge of this cliff for the upcoming 2015 season? Below is the list of running backs that lead the league in career carries, excluding, of course, Steven Jackson and Frank Gore, who were used in the aforementioned study.

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Three notable names appear on this list that surpassed the 1,800 carry milestone last season and are therefore in danger of suffering this infamous drop off the cliff (due to missing almost the entire 2014 season to suspension, Adrian Peterson crossed that mark in 2013).

Matt Forte (29 years old): 1,817 career carries

At the end of his eighth season in the league, Chicago Bears running back Matt Forte just barely eclipsed 1,800 career carries in 2014. In his two seasons under head coach Marc Trestman, Forte saw his workload and subsequently his production and efficiency in the ground game increase each year. With run-oriented John Fox now taking over the reins, there's little reason to believe Forte should see his workload decrease in the foreseeable future.

However, the potential fall over the 1,800 carry cliff still hangs over Forte's head. One saving grace for Matt is his prowess in the passing game, which may allow him to continue to contribute to the offense even if his ground production stalls.

Adrian Peterson (29 years old): 2,054 career carries

There are many reasons why teams should be cautious of the 30-year old superstar running back. Just recently, our Editor-in-Chief, JJ Zachariason, pointed out five very good reasons why the Cowboys should avoid AP altogether.

And now I'm here to provide you with one more strike against Peterson.

In 2013 -- the season before Adrian served what was essentially a 15-game suspension imposed by the NFL due to domestic violence charges -- Peterson crossed the 1,800 carry mark. His numbers from that season -- 1,266 rushing yards, 10 rushing touchdowns, and a Rushing NEP of 7.59 -- are eerily reminiscent of the average numbers for running backs right before they fall off the running back cliff.

Some may argue that Adrian Peterson is an otherworldly talent and that the rules don't apply to him. Indeed, his 2,000-plus yard rushing season in 2012 just months after suffering a gruesome ACL tear speaks volumes to the immense talents at his disposal. Yet, the risks remain. The swift and unexpected fall happened to Shaun Alexander, it happened to Clinton Portis, and it happened to Willis McGahee.

And for a player owed $44.25 million over the next three years standing on the very edge of this cliff, as an NFL general manager, betting on Adrian Peterson is not be the type of risk I'd be willing to take.

Marshawn Lynch (28 years old): 2,033 career carries

The parallels between Marshawn Lynch's and Shaun Alexander's careers are just too similar to ignore.

Both backs would cross the 1,800 carry mark during seasons in which their teams would make Super Bowl runs, falling just short of an NFL title. Following these seasons marking the apex of their careers, both running backs would find themselves under consideration as one of the best running backs in the league. And due to these conversations, both running backs would sign lucrative contracts that offeseason with Seattle. Both of these backs would also lose key offensive lineman going into the next season (Hutchinson for Alexander, Unger for Lynch).

But, in many ways Marshawn Lynch is also the polar opposite of Shaun Alexander. A hard-nosed and bruising running style on the field, coupled with a soft-spoken and unassuming personality off the field for the man they call 'Beast Mode' has endeared Marshawn to the hearts and affections of the fans comprising the 12th Man.

Still, in the face of the toll such a punishing playing style has likely taken on Marshawn's body, it's a fair question to ask how much longer he can keep up this elite production before he begins to break down like so many other running backs before him.

Conclusions

Using historical data, we've provided some predictability in the high stakes game of betting on running back longevity. It appears as though players plummet in productivity and value soon after the season they cross the 1,800-carry mark. Only time will tell if the high-profile names on our list of candidates will fall to this seemingly inevitable fate this upcoming season, but if there's one thing we've learned, its that the fall off the cliff is a swift and fast one.