Complex sentence: The two pairs of free-standing cable-supporting towers are high allowing shipping of head room under the main span, but most ships' captains prefer to pass through the unobstructed Drogden Strait above the Drogden Tunnel.
Split into simpler sentences: The two pairs of free-standing cable supporting towers are high allowing shipping of head room under the main span. Even so, most ships' captains prefer to pass through the unobstructed Drogden Strait above the Drogden Tunnel.