Baggage carts have traditionally been free in Europe, but not in the United States. Here's the reason.

When you arrive at Paris's Charles de Gaulle Airport, Fiumicino in Rome, or Heathrow in London with two weeks' worth of luggage, you can unload it all onto one of those ubiquitous luggage carts—free. In the United States, however, you'd pay as much as $3 a cart.

Carts are usually owned or leased by airports in Europe, while in the States they're owned by private companies. The largest of these, St. Paul–based Smarte Carte, negotiates a per-cart charge with each airport. It's ultimately the airports that set the price, however; at San Francisco International, for example, the carts are free to international travelers. Since the commission that runs the airport still has to pay Smarte Carte, it ends up absorbing the cost.

Until recently, revenues from ticket sales in Europe were so high that airports there could afford to provide free carts, says Ron Kuhlmann, vice president of transportation management consulting firm Unisys R2A. Also, European airports tend to be more service-oriented than their American counterparts. But with European airfares at an all-time low, airports are discovering that charging consumers for carts can add to their bottom lines. At Milan's Malpensa and Turkey's Antalya International Airport, for instance, Smarte Cartes now cost 1 euro (about $1.30) per use. (At Malpensa, the fee is just a deposit; many travelers don't return their carts to the dispensers, though, so the airport still makes money.)

Unfortunately for travelers, the days of free airport carts abroad may be numbered. According to Smarte Carte spokesperson Tammi Phippen, a handful of international airports are on the verge of signing contracts with the company.