The observant amongst you would have noticed that the overtake of the horses involved (amongst other things) a vehicle ahead of the motorbike, filming the bike rider through the overtake manoeuvre. The editing and the foreshortening effect of the camera lens may not give a faithful representation of what it really looked like or what actually occured.
The bike rider maybe 'knew' it was safe to overtake as the lead filming vehicle formed a barrier (of sorts) against oncoming traffic. Whether it was really the best example to have shown in what purports to be an instructional video is another matter entirely. At worst the bike rider followed the lead vehicle through on a (perhaps) dodgy overtake.... the classic... "I can't really see, but I'll let him be my eyes''....... Aieeeeeeeeee...... booger!
166
DO NOT overtake if there is any doubt, or where you cannot see far enough ahead to be sure it is safe. For example, when you are approaching
a corner or bend
a hump bridge
the brow of a hill
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AND well over a solid white line
165
You MUST NOT overtake
if you would have to cross or straddle double white lines with a solid line nearest to you (but see Rule 129)
129
Double white lines where the line nearest you is solid. This means you MUST NOT cross or straddle it unless it is safe and you need to enter adjoining premises or a side road. You
may cross the line if necessary, provided the road is clear, to pass a stationary vehicle, or overtake a pedal cycle,
horse or road maintenance vehicle, if they are travelling at 10 mph (16 km/h) or less.
[Laws RTA 1988 sect 36 & TSRGD regs 10 & 26]
The two horses were well under 10 MPH.
Whether or not the road was really clear you can debate at leisure.
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All in all, the two abreast horse riders didn't do themselves (or other road users) too many favours.