Over time, baseball teams develop identities, and even if the players change, the identities stay the same. The Yankees (and the Red Sox, for that matter) have cultivated a slow, plodding, methodical style of three-true-outcomes baseball that scores lots of runs and takes its time doing it. The Cubs, until recently, were the team who struck out the world when pitching and wouldn’t take a walk to save their lives. The Twins were all about manufactured runs and control pitchers.

The Mariners have an identity, based largely around 1-0 losses and the brutal, crushing inevitability of death. And some of this can be attributed to the players—this is the team that has tried Adam Kennedy as a designated hitter in recent years. But as Egon Spengler put it, “It's not the girl, Peter, it's the building!” Safeco Park has been absolutely brutal on offense. This has probably not had much of an effect on wins and losses—what Safeco takes way from offense it gives back to pitching and defense. So Safeco isn’t causing the Mariners to lose, it’s just subtracting from the sum total of joy and happiness in the universe.

So the Mariners are going to alter their fences to increase offense:

The fence will be moved in from 4 to 17 feet at different points in left field and 4 feet from straight center to the right-center gap. Additionally, the 16-foot-high hand-operated scoreboard down the left-field line will be moved back and no longer be part of the fence, so the outfield wall will be 8-feet high all the way around the park.

Jeff Kingston, the Mariners assistant general manager who was part of a committee that studied the situation, said the changes won't turn Safeco into a hitter's paradise but will even the playing field to a degree.

"We still think it's going to be on the pitching side of the spectrum," Kingston said. "Our approach all along was to make it more fair and be closer to the middle. We still want it to be a pitchers' park and build around pitching and defense, but we wanted to give hitters a chance to where if they really square it up and hit it 390 or 400-plus feet that they'll be rewarded."

This is what the fence change will look like:

Except for the somewhat larger change in left-center field, essentially the fence change covers what until now has been the warning track. So something like this Miguel Cabrera flyout would, with the new changes, have a chance to become a home run:

So how do we go about establishing the potential effects of this change on offense at Safeco? The Mariners say their internal study leads them to believe moving the fences would have meant between 30 and 40 extra home runs per year each of the past few years (that’s total, so home runs scored by the Mariners and allowed by the Mariners' pitchers). If we watch television broadcasts and count deep fly balls in Mariners games in 2012, we can come up with similar totals, depending on one’s judgment of whether or not a ball would have been past the new fences (admittedly a speculative endeavor). But is there another way to establish the change in home run rates by moving these fences?

Thanks to Kevin Johnson’s invaluable parks database, we have a record of how the fences at various ballparks have moved over the years. So what I did was pull the changes in park dimension in left field, left center, center, right center and right field for years where that data was complete, and then look at changes in normalized home run rate over that time. (A methodological note—I kept all teams where there was sufficient data, not just the ones that had fences move.)

What I found was that a change in fence distance was (not shockingly) a meaningful predictor of change in a park’s home run rate, after controlling for changes at the league level; combining all five distances in a regression analysis gives us an adjusted R-squared of 0.18, which means there’s a lot that’s not explained by changes in fence distance but a fair amount that is. If we apply our model to the proposed fence changes, using 2012 home run rates as our baseline, I come up with 22 additional home runs from the proposed change.

That’s, I think, a plausible number, and in the grand scheme of thing not too far off what the Mariners found or what looking at video says. The other interesting finding is that changes in fence distance seem to be a very poor predictor of changes in doubles rates, actually turning up a negative adjusted R-squared. (I’m sure some of you are thinking that this shouldn’t be possible, but adjusted R-squared is adjusted in such a way that allows for negative values, although that’s a good sign that you should be reconsidering what you’re doing.) So in essence, each of those home runs predicted by our model would seem to otherwise be an out.

Using linear weights, that means that 22 home runs come out to 37 total runs. Divide by 162 to get runs per game (for what we’re doing, we need to prorate these over team games, not home games) and then by half to get the contribution to the offensive team only, that’s .11 runs per game. The Mariners have scored 3.77 runs per game this season, the lowest in the American League; with the additional home runs, that goes to 3.88. If we assume that instead the Mariners pick up the high-end estimate of 40 home runs, that’s .20 additional runs scored per game, or 3.97. The next lowest-scoring offense in the American League, the Cleveland Indians, scored 4.14 runs per game. The fence change, as envisioned, provides a small boost to offense, but Safeco is likely to remain one of the most pitcher-friendly parks in the league, old fences or new.

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Doesn't the low humidity and temperatures of Safeco also reduce how far balls carry, similar to Pac Bell in SF?

Even if the fences looked like Newyork it's plausible it would still be a pitchers or nuetral park.
Don't we also have to add runs to the visiting team as well? Did I miss something? Thanks.
There is yet another way to approach this problem, using Greg Rybarczyk's home run records. Greg records the landing point of each home run as well as the height and distance to the nearest fence. One can use those data to estimate the effect of moving the fence in or lowering its height. Strictly speaking, this technique can only be used to investigate moving the fences out, in which case one then asks how many home runs stay in the park. But for small changes, I suspect one can get a good estimate of the effect of moving them in also. One looks at the distribution of fly ball distances and extrapolates inside the current fence position. This is exactly the technique I used to estimate the reduction in home runs expected at Chase Field if they stored the balls in a humidor.
Alan, I'm not sure I understand what you're saying well enough to attempt it, at least in terms of estimating the distribution of deep fly balls from Greg's home run data.
You can look at Greg's home run data to map out the distribution of fly ball distances relative to the fence (i.e., distance between landing point and nearest fence). Then extrapolate that distribution to negative numbers (i.e., fly balls that didn't make it over the fence). When I looked at this distribution across all of MLB stadiums, I found that a 1% change in fly ball distance was equivalent to a ~10% change in home run probability. You can apply that same kind of reasoning to the fence location. That is, move the fence in so that it is 1% closer and you will increase home runs by 10%. It might be interesting to compare that result, given the new fence distance, with your estimate.
One question that occurred to me about the total number of runs scored going from 3.77 to 3.88: would the extra 22 home runs hit by moving the fences in not also lead to further runs scored since there would be 22 fewer outs made on caught fly balls as opposed to those balls going over the fence? Perhaps the number is not quite 22 since maybe some of those were actually long singles or doubles or even triples, but I'm sure you get the idea.
Colin, are you assuming that those 22 extra HR are coming from outs, or from singles, doubles, and triples? The reality, I think, is that they come mostly from outs but also from some singles and extra base hits (mostly doubles and triples).

Also, with a larger field, you have the outfielders playing deeper, which means a few extra singles to the short part of the outfield and base runners advancing more on balls hit to the OF, so moving fences in is not going to increase scoring as much as we might think based upon the extra HR.

In any case, if we assume that the fence change will increase run scoring from .11 to .20 runs per game per team, and we split the difference (.15), then we increase the PF for Safeco by around .15/4.40, or .034.

I have Safeco with a PF (run factor) of .9 going into this season, based on data since its inception. That is the second lowest PF in baseball (Petco is .83).

So .9 to .93 is not that much really. .93 will still be the 4th lowest in baseball, after SD, OAK, and SF, again according to my PF database.

It is not easy to make a park with a large foul territory (around the 6th largest in MLB), at sea level, and cool temperatures into a hitters or even a neutral park.

And it doesn't really suppress HR all that much. If you look at the parks database that Colin references (Seamheads), you will see an overall HR factor or around .9 or so, which is not so extreme. For the average full time player for the M's, that is less than 1 HR per year!
MGL, I talk about checking for changes in doubles rates and not finding fence change a significant predictor for doubles. So I assumed that the additional home runs were all coming from the pool of outs. I suspect that will overstate the run impact slightly, but since the conclusion was that the run impact of the fence change is likely modest that won't change the conclusion any.