Reduced width traffic lanes

In order to make better use of existing roads, the lane width may be reduced so that more lanes are available on the same total road width. In principle, this is expected to increase traffic throughput, although with reduced speed. However, in practice, effects may be different. For instance, smaller lanes require a higher attention level of the drivers who may start showing signs of fatigue. This may compromise safety, inducing accidents. And accidents reduce traffic throughput. So, the question is under which circumstances narrow lanes do not compromise safety and increase throughput.