(25 Feb 2019, 5:38 am)Dan wrote Yes, all done by the manufacturers in build.

It's based on a number of factors I believe - not just the physical space but also the axle weight.

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Dependent on the vehicle type, there are a whole range of factors - One is the space (in square metres) allowed per standing passenger that determines how many standing are allowed.
For vehicles such as the Optare Solo it is governed by the following regulations regarding the vehicle approval process (it’s a very long document, 52C deals with the calculation of standing passengers, but the bus has to meet all the regulations to be certified for use).
https://assets.publishing.service.gov.uk...oaches.pdf