Strictly speaking, 'fewer' is used for items that can be counted, while 'less' is for something more abstract that cannot easily be measured. So in the context of 'people', 'fewer' would be correct.
However, 'less people' is often used and could be considered correct as well, depending on the context. In the context given, they may not intend to specifically measure the number of people using the warning lights, and are talking about people in a more general way. So in that sense, 'less' would be acceptable.
Very interesting. Thanks.
