i dont know if you can avoid it but if you put some oinions in with them that will keep them away i think
im not quite sure

well ive sown my carrot seeds now and as charlotte said mine r nxt to my onions. I grew them last year and had no problems with carrot fly, I do have them in raised beds tho. I read somewhere tht they are better higher. (dont quote me on tht mind)

yes you are both right. think I told Charlotte this, this morning. Some people also say put a barrier around the carrots 2 feet high to deter these flies; others say French marigolds helps to if planted near by