Lurkedforever1 - your dog doesn't. Fair enough. The 'my dog doesn't do that' attitude doesn't matter. On a public Highway I still think all dogs should be on leads for everyone's safety. Yours, your dogs, pedestrians and motorists.
I don't think there is anything 'hilarious' in expecting people to keep their dogs on leads. In fact I am pretty sure it is an offence to have your dog off a lead and can be fined for failure to do so if an area has a PSPO or you are near play parks. Public highways are not clear but you can still be fined if your dog is deemed out of control
www.thekennelclub.org.uk/media/8277/law.pdf
Also here are some 'handy pointers' as to why I keep my dogs muzzled and on leads
"Your dog is considered dangerously out of control if it:
injures someone
makes someone worried that it might injure them"
So, back to the OP, that dog would be considered out of control.
www.gov.uk/control-dog-public/overview
By keeping a dog on a lead you are reducing the risk sure he might injure them OR leave them panicked it might injure them. Last thing I want is someone panicked, calling the council and me risking my dog being taken away. Its worth thinking about.
Your dog might not do that but there is ALWAYS the possibility they could.
I don't think I am being ridiculous in saying leads, muzzles and training protects me, my dog, other dogs and pedestrians.
Dogs on leads also protects the environment. Keeping them leashed stops them disturbing ground nesting birds amongst other wildlife. As well as protecting other domestic and farm animals.
www.gov.uk/government/publications/the-countryside-code
I've included the links for other MNers who might want clarification on the rules surrounding dogs, leads, Public space protection orders and so on.