Possible Clues:

• Visiting sister, went and arrived with a progress check
• A Merc with ease passed on the outside activating this?
• What might capture escaped mare panicking?
• Roadside enforcer
• Reads pace, devised to catch one? It does
• Half-heartedly passed a Merc stupidly conserving energy -- one could be caught by it?
• Excessive on reflection, a Merc flying is caught by it?
• *Robocop?
• Roadside deterrent