For more than three years, the average one-way fare between Detroit and Philadelphia never dipped below $308, and sometimes moved higher, topping $385 at one point.
But then, early in 2016, fares suddenly started to fall, according to data from the Bureau of Transportation Statistics.
By the end of the year, the average one-way ticket between the two cities stood at just $183.
What changed? The primary factor was Spirit Airlines.
The low-cost carrier began operating flights from Philadelphia International Airport to Detroit.....