Blame John Mastick for this one.
It is one of Mark's early attempts to write a solid rock song in his late high school days when The Last Psalm was just getting started, and opening with the harmonics (always part of the song) led into the driving but repetitive chord progression. The lyrics, perhaps too obviously, were inspired by Robert Frost's The Road Not Taken, which had come to Mark through the Frostiana choral collection by (one of his favorite composers) Randall Thompson. The Christian overtones of the concept of choosing between two roads had not been lost on him or his peers, and often preachers would mention the notion in evangelistic sermons. Mark put the concept to music.
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Of course, in a rock band when you have a musically dull song you insert what you hope will be an interesting instrumental, and when The Last Psalm did this after the first batch of verses Jeffrey would go into an open-ended lead break over the vamping progression, then pass the lead to John on drums, who would play long enough--well, long enough that the rest of the band would step aside, find seats off stage, and wait for him to start to get tired. He loved that part, that moment in the spotlight. When the others thought he might be nearly finished, they returned to their places, and he cued them back in. Mark would play an open-ended lead solo over the vamp, and cue the vocals back in for the last block of vocals. It then transitioned into a different vamp (a sliding E ninth, a popular jazz chord) and Jeff would play another solo in an extended live fade. Once they had reached the point that they had faded out completely, John would wait a moment (generally waiting for the crowd to decide it was over), hit a pickup note, and the band would play the progression one more time into a thrasher ending. It was one of the crowd favorites back then.
When that band dissolved in 1975, Mark was trying to figure out how to use as many of its songs as possible in his anticipated acoustic soloist career, and for some reason wanted this one--but obviously the instrumental would not work, and without it the song was pretty dull. Working together with his friend and advisor Dave Oldham, he wrote an acoustic guitar break that owed much to Rick Wakeman, Yes, and Emerson, Lake, and Palmer. (The pause after the descending guitar frill before the driving power chord sequence was one of David's contributions, although the lead or bass melodic line with it was a much later addition when Collision was playing it.)
Thus when it came to Collision, it had this complicated guitar solo that was easily converted into a full band arrangement. Since John of course wanted the drum solo but Mark felt that such open-ended solos were too much a sound of the past, the solos were reconfigured into eight measure blocks, passing them from one musician to another.
Since John never played with the band, obviously he never played the new arrangement. Nick did, though, and the song made a couple of concerts in 2012. The instrumental was challenging, though, and it might be we never got it quite right.