Transport for London (TfL) does not hold details of the number of buses that broke down on the north and south sides of the Thames. Instead, TfL measures the percentage of service unavailable, defined as ‘lost mileage’. This metric incentivises operators to keep their vehicle fleets well maintained and have substitute vehicles available so they can provide as much of their scheduled mileage as possible, making them resilient to unforeseen events like a bus being withdrawn following a collision or a break down and ultimately providing better passenger services. The measure isn’t sub-categorised further into types of mechanical issues or location.
For the most recent full financial year of 2018/19, 0.31 per cent of network mileage was classified as mechanical lost mileage – indicating how little disruption was caused by any form of engineering issue.