For those who have visited London and wondered how they know their double decker buses won't fall, this apparently is how they will find out. According to police regulations, employees of the London General Omnibus Company put their 60-person bus through a “tilt test”. The test was considered successful if the double-decker buses would tilt 28 degrees without tipping over. There are sandbags in the upper deck to simulate 60 passengers.
Note the chains and light lip, these do not prevent the vehicle from tipping. The lip is to prevent the vehicle from sliding sideways on the ramp. The chains (straps these days) are slightly loose and will stop the vehicle from rolling as the brakes must be off during the test. They will also catch the vehicle if it tips, the test is to find if the tipping point is below the required limits, they don't need to fully roll the vehicle to find out. This would be quite an expensive method.
The first engine-powered double-decker bus appeared in London in 1923. At that time, there was a shortage of buses in London and various companies were competing for bus dominance. By 1924, there were more than 200 independent buses operating around the city, running along popular routes. These independent buses were known as "pirate buses" (although, unfortunately, they lacked peg-legged men with eye spots). Not chained to an official route, pirate buses would sometimes take side streets and alternate routes to get to faster destinations. It was because of competing bus companies that the London General Omnibus Company, the city's largest operator, painted its buses red to stand out from the competition. The Metropolitan Police approved the red buses; the color was so easy to spot acting as a warning to those crossing the street. The red double-decker buses in London would become a national symbol of England and the United Kingdom.