Why Travel Distance and Common Path Are Two Different Limits

Why this matters

A tenant layout can sail through the limit everyone checks and blow through two nobody checks, and the one everyone checks is travel distance. Travel distance asks how far the walk is. Common path asks how far you walk before you have a choice. Those are different questions about the same path, they fail independently, and the second one is the one that gets somebody killed, because a single blocked path with no alternative is the condition that turns a manageable fire into a trapped occupant. Adding another exit at the far end of the building, which is the fix everyone reaches for, frequently improves neither.

Two limits, two different questions

Exit access travel distance is measured from the most remote point of the occupied portion, along the natural and unobstructed path of egress travel, to the entrance to an exit. It stops at the exit, not at the outside and not at the public way, because past that point you are in protected construction. A sibling article covers why the segment boundary sits there.

Common path of egress travel is the portion of exit access an occupant must traverse before two separate and distinct paths to two exits become available. It starts at the same remote point as travel distance and ends earlier, at the moment a genuine choice exists.

Both are measured around the actual obstructions in the space in the adopted code, meaning around fixed furniture, equipment, casework and racking rather than as a straight line across a plan. A measurement taken with a scale rule on a clean drawing is optimistic by whatever the real furniture adds.

Because common path is a portion of the same walk, common path is always less than or equal to travel distance from the same starting point. That is a definitional relationship, not a rule of thumb, and it is worth holding because it tells you what a failure means: a plan that fails common path and passes travel distance has a topology problem, not a size problem.

The third limit, which gets confused with the second

A dead-end corridor is a portion of a corridor from which egress travel is possible in only one direction. It has its own limit and its own measurement, taken within the corridor from the closed end to the point where travel in a second direction becomes possible.

Common path and dead end overlap constantly and are not the same measurement. Common path starts at an occupant's remote position, including travel inside a room. A dead end is measured in the corridor only. A layout can produce a dead end without a common path failure and the reverse, which is why both appear separately in the adopted code's tables.

Where the numbers come from

All three limits are set by occupancy group in the adopted code's tables, and every one of them gets a longer allowance where the building is sprinklered throughout with a system installed per NFPA 13 in the edition the authority having jurisdiction adopted. Commonly cited values for a business occupancy in a sprinklered building are 300 ft travel distance, 100 ft common path and 50 ft dead end, against 200 ft, 75 ft and 20 ft respectively without sprinklers, and every one of those is read off the adopted and amended edition rather than from memory. The AHJ is a named role with legal authority under that code, not a synonym for whoever inspects.

On the narrower federal side, 29 CFR 1910.36(b)(1) requires at least two exit routes for the workplaces it covers, with a limited single-exit allowance where the number of employees, the size of the building and its arrangement allow all employees to evacuate safely. That is a federal floor about the number of routes; it does not carry the code's distance tables, and the two must never be quoted as if they were one requirement.

One gate, two outcomes: the same suite before and after a fit-out

A single-story sprinklered office suite, 200 ft east to west and 90 ft north to south. A spine corridor runs the length of the south side, with an exit at each end. Business occupancy. Limits in this jurisdiction's adopted edition: 300 ft travel, 100 ft common path, 50 ft dead end.

The gate is one question: at what point in the walk do two separate and distinct paths to two exits first become available?

Outcome A, open plan. The most remote workstation sits at the north-east corner. The occupant walks 60 ft south through the open area to reach the spine corridor. At that point they can go 20 ft east to one exit or 180 ft west to the other, and those are separate and distinct paths to two different exits.

• Common path: 60 ft. Limit 100 ft. Passes.
• Travel distance to the nearest exit: 60 + 20 = 80 ft. Limit 300 ft. Passes at about 27 percent of the limit.
• Dead end: none.

Outcome B, after the tenant builds private offices. A side corridor is added along the north wall, closed at its west end and connecting to the spine only at its east end, 95 ft away. The deepest office door sits 5 ft from the closed end, so 90 ft from the junction, and that office is 20 ft deep.

• Common path: 20 ft inside the office plus 90 ft along the side corridor equals 110 ft. Limit 100 ft. Fails, at 110 percent of the limit.
• Dead end: 95 ft measured in the side corridor from its closed end to the junction. Limit 50 ft. Fails, at 190 percent of the limit.
• Travel distance: 110 ft plus 40 ft from the junction to the nearer exit equals 150 ft. Limit 300 ft. Passes at 50 percent of the limit.

Two of three limits fail while the one everyone checks sits at half its allowance. Nothing about the walk got longer in a way that shows up on the measurement everybody takes.

Qualifier lines, printed:

• Measured around obstructions: the 60 ft in Outcome A and the 20 ft inside the office in Outcome B are both walked around the furniture as installed, not scaled off a clean plan.
• Travel distance endpoint: measured to the entrance to the exit, not to the exterior door beyond it and not to the public way.
• Common path is a portion of the same walk: 110 ft common path sits inside the 150 ft travel distance in Outcome B, and 60 ft sits inside 80 ft in Outcome A. The relationship holds in both, as it must.
• Dead end measured in the corridor only: 95 ft, excluding the 20 ft inside the office, because the dead-end measurement does not start in the room. That is why the dead end and common path figures differ by exactly the office depth less the 5 ft offset: 110 minus 95 equals 15, which is the 20 ft room depth less the 5 ft the door sits from the closed end.
• Sprinkler basis: the 300, 100 and 50 ft limits apply because this building is sprinklered throughout with a system per NFPA 13 in the adopted edition. Take the sprinkler system out of service and the governing limits become the unsprinklered row, at which point Outcome A's 60 ft common path is still passing but has far less margin.
• Adoption gate: all three limits read from the code edition this jurisdiction adopted and amended, for this occupancy group.
• No flattering rounding: 110 ft against a 100 ft limit is a failure, not "essentially at the limit".

Why adding an exit usually fixes nothing

The instinctive response to Outcome B is to add a third exit at the west end of the spine corridor. Run the numbers through the same gate.

From the deepest office, the occupant still walks 20 ft to the office door and 90 ft east along the side corridor before any choice exists, because the side corridor still has one end. Common path stays at 110 ft. The dead end stays at 95 ft. Travel distance to the nearest exit stays at 150 ft, because the new exit is further west than the one already being used. Three limits, none moved, and a new exit built.

The fix that works is topological. Open the west end of the side corridor to the spine with a short link, say 15 ft, landing 25 ft from the west exit. Now the occupant leaving the deepest office has a choice as soon as they reach the corridor:

• Common path: 20 ft inside the office plus 5 ft to the west link equals 25 ft. Passes.
• Dead end: eliminated, because the corridor has two ways out.
• Travel distance west: 20 + 5 + 15 + 25 equals 65 ft. Passes, and it improved from 150 ft as a side effect.

That is the whole lesson. Common path is set by where the second path becomes available, so it is fixed by connecting, not by adding. Travel distance is set by length, so it is fixed by shortening or by placing an exit nearer the remote point. They respond to different interventions because they measure different things.

Sibling-rule check, printed: all travel distances here end at the entrance to an exit, which is the segment boundary the means-of-egress article defines, and the corridor in question is exit access throughout rather than an exit, so nothing in this example credits protected construction it does not have. The occupant load that sizes these corridors is a calculated design number rather than a head count, per the occupant load article, and none of the three limits above is relaxed on the basis of how many people are actually present.

Effort, in labour terms rather than currency: measuring all three limits on this floor took one person about 90 minutes with a measuring wheel, walking the real paths around the real furniture. The design review that followed took a multiple of that. Doing the measurement before the private offices were framed would have taken under an hour on the drawings and would have caught both failures while the correction was a line on a plan rather than demolition.

When you walk it, keep the wheel and any tape out of the walking lane so you are not creating an obstruction in the path you are auditing, and do not prop a door open to run a measurement through it - a propped rated door is an impairment of that assembly for as long as it is propped, with a start, a compensating measure and an end.

How to verify a layout against all three

1. Find the genuinely most remote point, which is often not the one that looks furthest on the plan once furniture and casework are in.
2. Walk to the first point where two separate and distinct paths to two different exits exist. Record that distance as common path. If you never reach such a point before an exit, common path equals travel distance and the space is single-exit.
3. Keep walking to the entrance to the nearest exit. Record that as travel distance.
4. Go back to every corridor and find the closed ends. Measure each to the point where a second direction opens up. Those are the dead ends.
5. Read the limits off the adopted edition for this occupancy, and check whether the sprinkler row applies. If the system is impaired, the unsprinklered row is what the space is actually operating under for the duration.

References

• The model building code and fire code as adopted and amended by your authority having jurisdiction, which own the travel distance, common path and dead-end tables and their sprinkler allowances
• NFPA 101, Life Safety Code, in the adopted edition, which carries parallel limits and reaches existing buildings
• NFPA 13, in the edition the authority having jurisdiction adopted, for what "sprinklered throughout" means when a longer limit is claimed
• 29 CFR 1910.36(b)(1), the narrower federal requirement for the number of exit routes in general industry
• See related: What an Exit Access, an Exit and an Exit Discharge Each Are; What Occupant Load Is Actually Counting