The first call is not to the yard group chat
A horse loose on a road is one of the few yard problems that can kill a stranger, and that fact sets the order of everything. Call the emergency services before you run out of the yard: police can get a car to the scene, slow or close the road, and put out warnings, and minutes spent doing that while the horse is still trotting the verge are not wasted — they are the minutes in which a driver doing full speed round a bend meets a warning instead of a horse. Tell them the road, the nearest junction, the horse's colour and which way it is travelling. Only then grab the kit — headcollar, lead rope, and a bucket with a little feed to make noise with — and go, taking a second person if you can.
Split the job: traffic and horse
Two problems are running at once and they need two people wherever possible. One person becomes the traffic sentinel: positioned well before the horse on the approach side, clearly visible, using a torch or hazard lights in poor light, signalling drivers to slow with a flat, slow palm-down motion — and standing somewhere with an escape route, never in the carriageway itself with nowhere to go. The other person goes to the horse. If you are alone, the traffic call to police matters even more, and you work the horse from the verge side, accepting a slower job rather than stepping into the road. Drivers mostly cooperate once they understand; what causes accidents is the driver who gets no warning.
The catch: boring on purpose
Everything about the approach should be deliberately dull. Walk, do not run — a running human reads to a loose horse as predator or playmate, and both end with the horse moving faster. Come in at an angle toward the shoulder, bucket rattling gently, voice level, and let the horse make the last steps to you; horses that have escaped are usually more interested in grazing and company than in freedom, and most 'uncatchable' road horses were catchable until someone lunged at a headcollar. Slip the rope around the neck first, then the headcollar on, and lead from the shoulder with the horse between you and the verge, not between you and the traffic. Head for the nearest field gateway, lay-by or side turning to get off the road entirely — do not lead the horse back along the carriageway any further than you must, however close home is.
What never to do
Never stand in front of a moving horse to wave it down; it will go through you or over you. Do not grab at a bridle, rug strap or mane on a horse that is trotting past — you will be dragged or pulled into its path. Do not chase it in a vehicle, and do not let well-meaning drivers do so either; a herded horse panics and panicked horses jump fences, hedges and crash barriers. And once the horse is caught, do not simply turn it loose back into the field it escaped from until you know how it got out — walk the boundary first, or you will be repeating the whole emergency within the hour.
Afterwards — and the backup for next time
Once the horse is safe, check it over properly: road work leaves its mark in skinned knees, lost shoes, strained tendons and gravel in the hooves, and a horse that has galloped on tarmac deserves a careful leg check and a quiet day. Then fix the cause that day — fence, gate, latch — and tell the police the horse is recovered so the log can close. Finally, set the backup up before you need it again: in KinsLe, a Lost Pet Alert System entry for your horse costs nothing in its profile, is pinned to the map view, and lets whoever spots the horse in a lane at dawn reach you directly.