The Boston Celtics won 31 road games this year, more than any team in basketball. They became the first team since 2001 to sweep a three-game swing against the mighty Texas trio, beat seven of the other 12 playoff teams in their own building, and didn’t lose a road game by double figures until the last week of the season.

Then the playoffs started, and all that went out the window. Suddenly, the Celtics can’t win on the road to save their lives.

For a second straight series, Boston’s inability to win on the road has landed it in trouble in a series in which it was heavily favored.

The Celtics’ 88–77 loss to Cleveland in Game 4 on Monday is the latest sign that all may not be quite as well as thought in Paradise.

It was their fifth straight road setback in the playoffs, including three to the lowly Atlanta Hawks in the first round. Additionally, it was the second consecutive double- figure defeat — compared to the one they had in 41 regular-season road contests.

What’s going on here anyway?

A quick look shows that Boston’s offensive output has stayed remarkably consistent, whether at home or on the road. It’s not consistent at a terribly high level, mind you, but it hasn’t dropped off much when away from home.

Defensively, on the other hand, the Celtics’ mojo has mysteriously gone missing outside the TD Banknorth Garden. Boston has allowed 85 points or fewer in all six playoff home games, and in some cases drastically fewer — they held Atlanta, for instance, to 65 points in the clinching Game 7.

On the road, they’ve given up at least 88 points every time out, and in some cases drastically more — they allowed 108 in Game 3 in Cleveland, for instance, including a 32-point first quarter that had them down by 19 and pretty much dead to rights.

Differences between the home and road effort are abundant. The Celtics have forced 16.8 turnovers a game at home, compared to just 12.4 on the road. They also foul more, give up more offensive boards, and allow more 3-pointers.

But actually, that’s a minor part of the big picture. The main reason the Celtics are struggling on the road is simply because their opponents are making many more shots. Having watched several of the games, one can argue the quality of those shots hasn’t been markedly different. But Boston opponents have shot 48.3% in five road games and just 34.5% in six home games. The disparity on 3-pointers is even greater, going from 39.2% on the road to a meager 25.3% at home.

However, that’s just one angle from which to view the Celtics’ surprisingly bumpy postseason.

Another is to combine all the numbers from both home and road and look at the big picture, and that presents a different, simpler story: Basically, these guys are fouling like crazy.

The average team allowed .306 free throw attempts per opponent field goal attempt during the regular season; Boston, while it excelled in the other aspects of defense, had the league’s fifth-worst mark at .341.

In the playoffs, that number has gone further south — all the way to .412. It’s been consistent in both home and road games, too. They’re fouling even more often than the league’s most foul-prone team, Utah, did during the regular season.

It’s not like their opponents were foul-drawing aces, either. Atlanta admittedly was above average in this regard, ranking sixth in the league, but Cleveland was right around the league average. Nothing in their marks would suggest that Boston’s foul rate would suddenly rise by over 20%.

It’s been doubly punishing for the Celtics because both these teams weren’t any good offensively in the regular season. Atlanta finished 16th in Offensive Efficiency (my measure of a team’s points per 100 possessions), while Cleveland was 19th; they’re two of the four worst offensive teams in the playoffs. Basically, if the Celtics could avoid fouling them, they’d probably shoot a brick.

Even with all the fouls, Boston is no. 1 in playoff Defensive Efficiency, but it’s not the overwhelming no. 1 that it was in the regular season.

And that, in turn, has exposed problems with an offense that even at its best wasn’t overwhelming.

In particular, wings Ray Allen and Paul Pierce are struggling mightily. Pierce is shooting only 41.2% in the playoffs, while Allen is at 39.7%. Neither is getting to the line much, but the two are still combining for more than eight turnovers a game.

What’s so striking in Allen’s case is that he has a matchup against a player , Cleveland’s Wally Szczerbiak, who is overmatched defensively and hasn’t been able to take advantage in any of the four games.

He has to make the Cavs pay for playing such a feeble defender on him, because Wally World can light it up at the other end. But Allen’s 12.97 playoff PER has been a huge disappointment.

Pierce has it a little tougher because LeBron James has been guarding him, and the Chosen One has become a very capable defensive player. But one also wonders if he’s still bothered by some back problems he suffered early in the Atlanta series, as he seems unusually inept at taking his man to the basket — which is normally his greatest strength. His playoff PER is a middling 15.22.

Of course, one always has to caution that we’re dealing with extremely small samples in the playoffs.

Boston has played 11 games and won six of them, and five of the six wins were by huge margins.

Divining the difference between a fluke and a pattern is as much art as science, so it may be inappropriate to sound the alarms just yet.

On the other hand, the Celtics have struggled against two opponents that allowed more points than they scored in the regular season. So even if they survive against Cleveland, what happens when they face somebody who’s actually good?

Obviously, the Celtics need to score more and foul less. If not, one of the most dominant defensive regular seasons in memory will be up in smoke — and for the third straight year a team that lost fewer than 20 regular season games while roaring to the league’s best record will fail even to win their conference, much less the championship.

jhollinger@nysun.com