The report shows that more people have switched from buses to cycling than have moved to trains or taxis.
The revelation comes with traffic congestion in cities like Glasgow reducing bus speeds to a walking pace, which industry chiefs have described as “frightening”.
The Confederation of Passenger Transport (CPT), which represents bus operators, gave “unprecedented” access to industry data to consultants KPMG for the research.
The study shows cars were the greatest cause of bus decline between 2011-12 and 2015-16 followed by online shopping, slow bus journey times and increasing fares.
However, it also calculated that 800,000 bus journeys were lost to bikes over that period. That compares to 200,000 journeys being switched to taxis, 100,000 to trains and 400,000 being lost due to increased flexible working. CPT admitted it was a “surprising statistic”.
Buses remain by far Scotland’s most popular form of public transport, accounting for 76 per cent of journeys.
However, total bus journeys dropped by 27 million over the four-year period to 409 million.
The loss to bikes is likely to have increased further because the bus decline has since accelerated, with the total estimated to have fallen by a further 16 million to 393 million last year.
Among the major factors are that cars accounted for 14.6 million fewer journeys, shopping and working online for 7.3 million, slower journeys for 5.9 million and rising bus fares 4 million. Economic and employment factors were other contributors.
These negative factors were offset by population growth adding 8.9 million journeys and better buses 2 million.
Colin Howden, director of transport campaigners Transform Scotland, said: “Bus journey times have been seriously impacted by traffic congestion in recent years. As bus services become stuck in queues of car traffic, it’s not surprising to see some passengers choose to walk or cycle instead.
“What is needed is action to limit the volume of car trips in congested urban centres as only this will allow the return of efficient and attractive bus services.
“It continues to surprise us that successive governments have not made the bus more central to our national transport policy ... all of this in a country with no car manufacturing industry for government to placate.”
Keith Irving, chief executive of the Scottish Government-funded cycle promotion agency Cycling Scotland, said: “The increase in cycling in Scotland is good news for our health, environment and economy, and spend on cycling delivers a fantastic return on investment.