There is an excellent book which covers planting distances of allotment crops by Caroline Foley, who basically says it depends on the crop; most crops will grow into whatever space they have, and staggered rows planting gives the best yields in raised beds. She also emphasised the importance of intercropping, using quick growing crops inbetween slow ones and most importantly having a succession of crops ready to put new ones in as soon as one is harvested. Plus using height, growing upwards not just straight onto the soil to make use of every inch of your growing space. I started off growing most things too close together, but things like carrots and leeks I use half grown alternate plants rather than chucking thinning away and then just keep doing that until they are more widely spaced and grow to full size.
