Off the OP topic, but locking gates is also related to potential liability due to 'attractive nuisances' laws. Horses, like swimming pools, are generally considered 'attractive nuisances' since they attract people (everyone loves horses, especially children) that may not be/are not aware of the dangers, e.g. The attractiveness of petting a horse may lead children to enter a fenced pasture because they don't realize the danger involved with the action. The laws state that the owner must take reasonable care to protect against such dangers, and the more that you do (e.g. Signs, locks, etc), the better you are protecting others from the danger and yourself from potential liability problems.