So here's the other way over the most southerly bridge of the three (four if you count the motorway).
Last picture first as on the eastern side there's a handy map showing how to do it. South is at the top which is confusing and it's only in Galician.
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It says pedestrians keep right. We will anyway so no worries.
This photo is at the first arrowhead I've drawn on the picture above:
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You can choose to go straight on to the highway sliproad (100m shorter) or turn right to follow a country lane. It looks like the access road to a naval facility but there's a sharp left opposite the gate.
Here's the beginning of the bridge, through a deserted petrol station.
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The other end looks like this.
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The X is where the sign is. It suggests you follow the cloverleaf under the road and up the other side. I suggest crossing to the left side of the road if the traffic allows.
Follow this straight road for another kilometre until you get to this point (the only café in Spain I've had to pay for crisps) where you intercept the camino once more.
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So, this shortcut is 3.5km. Via the official route it's approximately 12.5km, so 9km saved!