BERLIN — The summer Olympics gave Britain not only a psychological lift, but a spate of free, public Wi-Fi networks as well. In preparations for the Games, the operators O2 U.K., Virgin Mobile, BT and The Cloud carved up public venues across the British capital, broadcasting open Wi-Fi networks in major public squares, airports and subways and on train platforms.

Free Wi-Fi service is becoming an important tool for operators in maintaining quality mobile service during a time of skyrocketing data traffic, congested mobile networks and, for travelers, costly roaming charges.

Public Wi-Fi networks have sprung up in many big cities. According to Informa, a research firm, the number of Wi-Fi hot spots worldwide will reach 5.8 million by the end of 2015, up from 800,000 in 2010. Eight of the world’s 10 biggest mobile operators are using Wi-Fi to offload their cellular data traffic.

In New York City, free public Wi-Fi is available in 20 parks from AT&T. Google and Boingo, a maker of Wi-Fi equipment, sponsor free Wi-Fi at five New York subway stations. The city of New York is broadcasting free Wi-Fi from 20 pay phones.