many years ago CalTrans decided that they were switching to the metric system. Contractors and consultants started submitting design plans and other documents with metric units. But they also saw a huge increase in errors in the plans. When they investigated they found that everyone was working in imperial units and then doing unit conversions before submitting the final plans. This resulted in a lot of rounding errors (which are usually not too significant in CE/Construction) and flat out conversion calculation errors as well.