It seems obvious to me that if drivers are passing closer, then there's less room for error, either by the driver not judging distance or the cyclist needing to

It seems obvious to me that if drivers are passing closer, then there's less room for error, either by the driver not judging distance or the cyclist needing to avoid a pothole etc. There's also the intimidation factor - a lot of people are put off from cycling due to not feeling safe, and having vehicles pass at speed within a metre is extremely scary.