You are getting a hard time on here @WellThatsMeScrewed and FWIW I agree with you. You can always depend on (some) mumsnetters to be contrary all the time though. If you said 'AIBU to think it's unacceptable for a 4 year old to walk 7 miles to school' someone would say it's OK. (If they can walk, they can walk to school!)
I still don't understand why she can't use public transport tbh, you haven't been clear on that (unless I missed it.) But yeah, I absolutely would NOT be allowing my daughter to walk 3 miles, along lonely pathways and dual carriageways. Anyone who says it's OK for a child to walk along a dual carriageway is taking the piss. You can bet they would never let THEIR kids do this, and would very likely not do it themselves to get to work.
Very VERY few people would walk to work if it's 3 miles or more. If they couldn't drive/had no car, they'd get a bus, get a train (if the fuckers were actually running!) or cycle, or cadge a lift. Like fuck would most people be regularly doing a 6 or 7 mile round trip WALK to work and back.
As for the teachers/the school saying 3 miles is OK for the kids to walk to school. Do THEY walk to school??? Nah thought not. I have never known any teacher WALK (to the school they work at,) in my LIFE. Bet they all/mostly live 7-10 miles or more away. Convenient.
Shit like this makes me SO glad mine has left school, and is grown, and I don't have to deal with SCHOOLS anymore. We had to tolerate SO much shit from DD's schools. Primary AND secondary. So much bullshit like this, and so many rules and regulations and many that were utter pointless bullshit.
You have my sympathy. Hope things get better. 