Steven Jackson has been a reliable, productive back for quite a few years. But he doesn't have that much left in the tank.

There are a million different analogies that analysts and writers can use to illustrate various concepts and ideas about the NFL. But one of the most tired, yet accurate, is comparing a running back to a car. I mean, the position as a whole brought this comparison upon itself by allowing Carnell Williams to call himself Cadillac.

So forgive me for going back to the well and dipping into this analogy once again. I'll try not to say "he doesn't have a lot of tread left on his tires" only two or three times. I promise. But I think it can help paint a picture of the running back life cycle, and help give some insight into Steven Jackson as he enters the 2014 season.

Cars are a frustrating blessing. Most Americans would be lost without their cars (except for those who live in the tightly compacted major metropolises, and even then, a car is often a nice thing to have for road trips or making a quick run across town at an inconvenient mass transit time), but also find them to be one of the biggest drains on their bank account and sanity.

You can buy a new car every few years, but you're shelling out a lot of money that could go to other things in order to afford that constant stream of new car smell in your life. Most people opt to ride out the car they have until it's dead or dying, which for a select few, means a long, healthy life with only the occasional bump and bruise along the way.

The parallel to running backs here is obvious. Teams can shell out draft picks or cash to go get a new back every couple of years, or they can stick with their bell-cow for years and years, hoping he stays in one piece. But only so many teams have been able to get a long, productive life out of their backs.

The Running Back Used Car Lot

Since 2000, only 25 backs over the age of 30 have received a cumulative total of over 300 carries during their age-30 season or later. Of those 25, only 10 had 1,750 or more carries while 29 or younger. Of those 10, three are Hall of Famers (Emmitt Smith, Curtis Martin, Marshall Faulk), one is a lock to become one when heâ€™s eligible (LaDanian Tomlinson) and the rest are a whoâ€™s who of running back history of the 1990s and 2000s (Fred Taylor, Corey Dillon, Ricky Williams, Jerome Bettis, Ricky Watters, Eddie George).

The aforementioned Steven Jackson was well over that threshold of carries by the time he turned 30, putting him on the same workload level as Tomlinson and Martin before hitting the post-warranty portion of the running back life cycle.

So what does that mean for Jackson? Well, it means that unless he's Emmitt Smith (he's not) or Curtis Martin (probably not), he's going to struggle to get another 600 to 1,000 carries in his career, based on recent history. George and Watters failed to eclipse 450 carries after turning 30, while Faulk and Tomlinson both received about 500 more attempts before calling it quits. Jackson already has 156 carries on his post-30 tires, a feat that only 45 other backs have accomplished since 2000.

And unfortunately for Jackson, that means taking away his biggest asset: volume. Jackson was never a particularly efficient runner when considering numberFire's Net Expected Points (NEP) data. He started off strong, with a good season in 2004 on limited carries and a very good 2006 season, which was his only positive Rushing NEP season during which he carried the ball 250 or more times. That's something that Tomlinson, Faulk, Martin and even the man Jackson replaced in Atlanta, Michael Turner, did more than once in their careers.

So he got by as a fantasy asset and as a starting runner because he could handle the strain of being an every-down back in a relatively un-heavy system. The Rams ran the ball at least 400 times every year since 2006, keeping a consistent pass-to-run ratio during Jackson's tenure as lead back. Opportunity, rather than pure merit, was the reason why Jackson was so highly touted for years. He did well considering his circumstances, but his overall numbers were nothing to marvel at.

To be fair, most high-volume backs have negative NEPs due to the uphill battle they face to stay ahead of the down and distance when being focused on by defenses. But the lack of breakout seasons in Jackson's history shows us that he's not Curtis Martin, and he's not LaDanian Tomlinson. Which means we can't expect a lot from him in his post-30 seasons.

Figuring Out the Falcons' Carpool

But does that mean Jackson's time as a fantasy football relevant player are over?

Not necessarily. The Falcons got the same level of per-play NEP production as the Rams did from Jackson at the tail-end of his time in St. Louis. His -12.67 Rushing NEP for the season is on par with the per-carry production from his 2008 and 2010 campaigns, meaning he didn't really take a huge step back with the Falcons last year. But he did miss time due to injury, and that might have proven just how ordinary he was as a runner.

Teammates Jason Snelling and Jacquizz Rodgers did as much on a per carry basis as Jackson did during his limited time in the backfield in 2013 for the Falcons, all behind an admittedly really bad offensive line. All three backs posted Rushing NEP per carry totals in the negatives, with Rodgers posting a slightly superior average per attempt than Snelling and Jackson. And while the blocking will be somewhat improved in 2014, there's still reason to be concerned about Jackson's status as the unquestioned number one in the ATL.

Jackson figures to get "his share" of touches in Atlanta this season, because he's still an NFL-caliber player. He didn't fall completely off the map because he turned 30, but he's rapidly approaching the point of no return for backs. But what is a fair share of touches for a past his prime back who was never a special, transcendent back who's on the wrong side of 3000 career touches?

Especially when the team has added a running back worth considering, as the Falcons did in the form of FSU product Devonta Freeman, who our own JJ Zachariason wrote about here. Between Freeman and Rodgers, there will be plenty of competition for touches on an offense that's become more and more pass-heavy over the past two seasons.

So while the Falcons may keep Jackson around and give him a few carries now and then, it's only a matter of time before he's hurt again, or proves to be less efficient than his younger teammates who have more tread left on their tires. (Sorry, I had to do it at least once.) That means for Falcons fans and fantasy owners, it's time to let go of the keys, and start figuring out which of the newer models you prefer in Atlanta. Because Jackson's time as a leading back and a reliable producer are almost certainly over.