This is a hard one, because only you know your dog and even the breeds that are known for having and learning really good recall (which Cav's unfortunately aren't known for!
), there are some that won't be trained right and some that just don't at all. It is more a personal dog and training thing I think.
With that said though, I only let Brooklyn off at a fenced in park...it is HUGE though, which is wonderful for her. It has a baseball field and two soccer field on it...massive. And all dog owners in the area go there so the gates are usually shut (though one woman opened the gate for Brooklyn the other day, and I screamed at her running so fast and she just looked at me and said "she looked like she wanted to get out" :sl*p: Thankfully Brooklyn stood right there and waited for me to get her.
I also take her to an oval at the top of our street too, it does not have a fence, but it is on the water and far from the road, so really hard to bolt in.
Though, I train and practice with Brooklyn ALL THE TIME. I always carry chicken with me if we are going off lead and give it to her every single time she comes to me (even if it is just to say hello, so that she thinks "mama=chicken" every time. I would never take her off lead walking around or on the road or anywhere other than our places, and only when playing fetch...so her mind is engaged in "fetch", I would never just roam around with her off lead where she will be stimulated by something more interesting and off she goes!
Regardless, you need a good recall for emergencies, so work hard at it. And a tip...every time she comes back to you, treat her (with a high value treat like chicken) and also just grab her collar and then let it go again. Do this over and over...that way you desensitize her to you grabbing for the collar since that is part of the "getting of a treat". This is important because in case of emergencies, you don't want her to shy away or run when you reach in fast for the collar. Also, she will then think that coming to you doesnt automatically mean "we are leaving" if you do it over and over while off lead.
Point of story, I guess fenced in is the way to go. Gives them off lead time and you will always be safe
Check your local council or counties websites for off lead areas and fenced in areas available to dogs. Dogs won't tend to run away from other dogs either, so it might be good to go somewhere that other dogs are at vs. alone.
Oh, and when Brooky was young, I kept her on a really long lead at the dog park too...then eased into off lead. Fetch is too much fun, so enjoy yourself and just keep her safe and trust your gut.