The classic vision of a double-decker bus (one with two levels of seating) is the distinctive bright red buses of London, their spiral staircase and hop-on hop-off style transportation common in movies
. Although this design is rarely seen in public transit across the world, there are select cities that use them, as well as long-distance coaches and sightseeing tours (but those tend to be open-topped).
London’s open-platform double-decker buses, originally designed to carry the largest amount of passengers (60 to 80) without exceeding maximum vehicle length, were retired in 2005 mostly due to the inconvenience for disabled passengers. There are still some Heritage routes that use these buses, however. Newer double-deckers, more accessible for the disabled, continue to be in use across the UK.
Other countries that use the double-decker style of bus tend to be European or old British colonies, but there is a smattering of them across the globe.
Some cities first imported these buses directly from London, presumably admiring the design, including Davis, California (there are only six) and the original double-deckers in Colombo, Sri Lanka.
Double-decker buses vary in seating in different countries, for instance Berlin, Germany, where it can exceed 100 passengers, depending on when the buses were made.
This style of bus is extremely common in Hong Kong, after being introduced in 1949. Mumbai, India has been operating buses based on London’s design for even longer; they were first put into circulation in 1937.
They can also be found in Singapore; Dunedin, New Zealand; Las Vegas, Nevada (along the Las Vegas strip); and the only place I’ve had personal experience with them (excluding tour buses), Victoria, British Columbia.
In 2000, Victoria began using double-deckers imported from the UK in its public transit system. Ever since then, locals and tourists alike have celebrated them. In Victoria, even on the busiest routes, double-deckers tend to be less crowded than the average bus. The back of the first level of the bus has seats facing each other, designed for more of a conversational atmosphere than the usual rows. (If the buses were crowded, this would mean knocking knees with strangers and pointedly avoiding eye contact, but usually you can simply sit upstairs instead.) Stairs leading up can be awkwardly bent in an angular-type spiral, or wide and easily conquered. The upper level is the luxury level, which is why catching a double-decker can make a regular’s day, especially when it’s hot.
The upper level varies slightly from bus to bus. All, in my experience, have small air conditioning vents that you can aim at yourself or turn off (the upper level has another system of air-conditioning, however, that the passengers have no control over), while some also have lights you have the option of turning on. These buses offer an excellent view, and usually more space to sprawl out. On rainy days, there’s no need to squint through muddy windows on the upper level, and on hot days you can relax in the air-conditioned bliss without being stuck to the seat or the person beside you.
From Wikipedia and my own meandering experience.
Thanks to Albert_Herring, for the correction. :)