How do you let your special needs child become independent without stressing out?

My daughter with Aspergers wanted to walk over to a friends house. Her older sister was already there and couldn’t go with her. She wanted to go on her own and knows the route well. She’s high functioning so I didn’t think that it would be a problem. But I found myself finding every excuse to not let her go on her own, the biggest is that she would be on a busy road. Much to my relief my older daughter called and asked me to come over for something, thus alleviating the stress of letting my Aspberger daughter walk on her own.

She’s 11 years old, and I feel that I still don’t have the faith in her to be independent when she often doesn’t act that way in certain circumstances. I do let her be home alone when she’s secure and stable for an hour or two at a time. But other than that I want to be around or have her sister there.

How can I let go and let her grow more?