The problem isn't so much is your daughter mature enough, the real question is your daughter capable of protecting herself should something happen to her. The sad reality are there are a lot of pedophiles in our society and young children are at a increased risk when away from the home. While she may be able to avoid cars and may know not to talk to strangers the reality is these criminals are on the look out for situations such as this and can and will use this as a means to satisfy there own sick urges.
Children are snatched in broad daylight walking a few houses down the street........at a much older age then your daughter. A 1/2 mile bike ride unsupervised is to risky for a child this age.
Add to the fact that should something happen to her such as a accident or injury you could be charged with child neglect or child endangerment. While there is not a great deal of written law the fact is you can be charged with endangerment if the courts feel your direct actions caused harm to the child. At the least you risk a concerned citizen calling the police and charges stemming from that.
I would strongly caution against allowing her to ride alone. The fact is we generally tell parents that until your child is large enough to fend off a attacker they should be supervised at all times away from the home.