
And if you 'zap' her a nano-second too soon or too late

what then? She will make the wrong association and perhaps you'll
ensure she does take off onto a road because the
pain felt was at a time she saw/heard/sensed something or someone else or because she is fearful of you :(
This is a training issue - plain and simple. You get out what you put in and just because the behaviour
appears to be seasonal doesn't mean you shouldn't try and work through it. If you can't find a method of getting her undivided attention then don't let her off lead where you are currently. Source another park, field, woods, beach, moors, bridle path, TENNIS COURT if necessary which doesn't have open access to dangers and where she can be safely loose running.
Failing that play ball games, frisbee, hide & seek in the garden to let her run off steam and then keep her on lead for walking :)