FIFO Lane

FIFO lanes are the less-famous cousin of supermarkets in the pull-system toolkit. They solve the same problem (creating a capped buffer between two operations) for the cases where supermarket stocking does not work. Custom parts, perishables, one-off orders, anything where you cannot stock multiple instances of the same variant. The FIFO lane lets those parts get a pull mechanism while still enforcing flow.

"A FIFO lane is a queue with a cap and a rule: first in, first out, no exceptions."

How a FIFO lane works

The mechanism is a sequenced location between two processes with a fixed number of positions, often a roller track, a slotted rack, or a marked floor lane. Parts enter the lane in the order the upstream process produced them and exit in the same order to the downstream process. The capacity of the lane is the WIP limit for that part of the value stream.

The cap is what makes it pull. When the lane is full, the upstream process must stop, because there is physically nowhere to put the next part. The constraint forces the upstream operation to wait until the downstream consumes a part, which is the pull signal. When the lane is empty, the downstream process pauses, because there is nothing to consume. The empty lane is the signal that the upstream is falling behind, and either capacity needs adjustment or the takt is wrong.

FIFO lanes get used when supermarket stocking is impractical. Supermarkets work for parts that are stocked in identifiable bins and pulled by variant; you take whichever bin you need. FIFO lanes are for parts that cannot be substituted. Custom orders where each part is unique. Perishables where the older part has to be used first. Sequenced builds where order matters. In all those cases, the lane lets you have a small capped buffer without trying to pretend the parts are interchangeable.

The discipline is strict ordering. Once a part enters the lane, no other part jumps ahead of it. The downstream operator does not pick the easiest part to work on; they take whatever is at the front of the lane. A FIFO lane that gets reordered in practice is just a buffer with a name on it. The whole point is that the physical layout makes reordering impossible.

Where a FIFO lane fits on the shop floor

Picture a small fab shop running custom steel parts. About 30 percent of the shop's work is one-off custom enclosures, each with a unique configuration. The remaining 70 percent is repeat runs of standard parts. The shop has set up supermarkets between operations for the standard parts, but the custom work does not fit a supermarket model; each part is its own thing.

The shop installs a FIFO lane between the weld station and final inspection, sized for five parts. The lane is a five-position roller track on the floor. Welded custom parts enter at the back. Inspection pulls from the front in the order they arrived. When the track is full, the weld station stops on custom parts and switches to standard runs (which still pull from a supermarket). When the track is empty, the inspector pauses.

The result is a capped, ordered buffer that works for the custom parts without trying to stock them. Lead time on custom parts becomes predictable because the lane is short and the WIP is capped. The standard parts continue to flow through their supermarkets. The shop runs two pull mechanisms in parallel: FIFO for custom, supermarket for standard. Each variant gets the right mechanism for its demand profile.

Common mistakes with FIFO lanes

• No cap. Without an explicit WIP cap, the lane becomes an unbounded queue and pull breaks. The cap is the entire point.
• Reordering inside the lane. Pulling easier parts out of the middle defeats FIFO. The physical layout should make reordering impossible.
• Using FIFO when a supermarket would fit. If parts can be stocked by variant, a supermarket gives more flexibility. Use FIFO only when stocking is not practical.
• Sizing the lane to the worst-case spike. A long lane hides flow problems. Keep it short, just enough to absorb normal variation.
• Not pausing downstream when the lane is empty. An empty lane is a signal that upstream is behind. Pausing surfaces the problem; faking the work hides it.

FIFO lane and related Lean tools

A FIFO lane is the alternative pull mechanism to a supermarket, used when parts cannot be stocked by variant. Both are forms of pull system that cap WIP between operations. FIFO lanes are especially common in sequenced pull setups where each part is unique. The cap on the lane is a kanban-style signal in physical form: a full lane stops upstream, an empty lane pauses downstream.