There's a perceived safety issue that can exist in persuits like building and maintaining our own machines from freely available plans.
How many people does it take using a machine for how many hours before a machine is safe?
According to this there are 45.5 million registered drivers in the UK (2015):
https://www.gov.uk/government/publications/how-many-people-hold-driving-licences-in-the-uk
And average mileage is around 7900 (2013):
https://www.bbc.co.uk/news/uk-england-28546589
...and 1713 road fatalities in 2013 (including pedestrians, horse riders, cyclists, passengers, bus passengers, etc:
https://www.gov.uk/government/publications/annual-road-fatalities
All that and improving road safety is still a valid exercise.
