How many candidate alignments should a corridor feasibility study compare?

There's no code section that sets the number, so the honest answer is: enough to cover the real tradeoffs, and not one more than you can compare on paper before survey.

In practice that tends to land between three and six alignments carried forward into the formal feasibility comparison, pulled from a longer desktop list that might start at eight or ten. The long list gets built fast, often from little more than a topo sheet and a gut sense of where the terrain cooperates. The short list is the one that goes in front of the steering committee, and that's where the number matters.

Where the number usually lands

Too few alternatives and you haven't demonstrated due diligence. An agency reviewer, or your own PM, will ask why the obvious third option along the ridge was never scored. Too many and the comparison matrix stops being useful. Past five or six alignments, reviewers skim instead of read, and the team doing the scoring starts cutting corners on the criteria that actually distinguish the corridors, like slope-driven cut-and-fill or built-up area conflicts.

A workable rule: bring forward every alignment that is materially different in at least one constraint dimension, land cover, slope, or built-up footprint. If two candidates only differ by a few hundred feet of tangent and score the same on every constraint, you don't have two alternatives. You have one alignment with a rounding error.

The other driver is how much the owner is willing to spend narrowing further. Each alignment you carry into the matrix costs staff time to score consistently, and if field verification is in scope at this stage, it costs survey budget too. Six alignments scored with real rigor beats ten scored with shortcuts.

Building the alternatives analysis matrix

The matrix itself is usually a spreadsheet: rows are constraint categories, columns are candidate alignments, cells are either a pass/fail flag or a weighted score. The categories that show up on nearly every corridor project are slope (grading cost, drainage, erosion risk), land cover (clearing, permitting exposure, environmental review triggers), and built-up areas (ROW takes, utility relocations, community impact).

The spreadsheet itself is easy to build. Getting consistent inputs for each row is where projects break down in practice. One engineer eyeballs slope off a contour layer for alignment A, another pulls it from a different DEM for alignment C, and now the matrix is comparing apples scored by two different people on two different days. The scoring has to come from the same source, read the same way, for every alignment in the matrix, or the ranking isn't defensible when someone asks how you got there.

That's the piece a desktop GIS pass struggles to deliver fast: a single constraint-scored view of every candidate, built from the same imagery and the same masks, so alignment A and alignment E are comparable on the same terms. If your team is assembling that by hand for each corridor, overlaying separate slope, land cover, and built-up layers alignment by alignment, a constraint-scored map of the full candidate set built at project start gets you to the same comparison without the layer-stacking.

Narrowing before survey crews mobilize

The point of all this is to spend survey money on the one or two alignments that deserve it. A field crew walking a corridor that the constraint matrix already flagged as the worst option on slope and built-up conflicts is a sunk cost nobody wants to explain in the project close-out.

So narrow in two passes. First pass: cut anything that fails a hard constraint outright, a wetland crossing with no feasible avoidance, a slope that pushes earthwork past what the budget tolerates. That usually takes the long list down by half before anyone opens a scoring matrix. Second pass: score what's left against the weighted criteria and carry forward the two or three that are genuinely competitive, where the final pick depends on something survey and geotech still need to confirm.

If that second pass is the step eating your schedule, it's worth seeing what a corridor-wide constraint map looks like before your team builds another one by hand.