Dock Appointment Backlog at a Cross-Dock
A regional cross-dock handles 140 inbound trailers and 130 outbound trailers per 24 hours across 18 doors. Doors are shared: each door can serve either flow, but switching a door from inbound to outbound takes 25 minutes of setup. The facility runs two 10-hour shifts with a 30-minute handover. Last month, average inbound trailer dwell was 3.1 hours against a 2-hour target, and 22% of outbound trailers missed their scheduled departure. The dock scheduler reports that inbound arrivals cluster between 06:00 and 10:00 (about 55% of daily volume) while outbound departures cluster between 14:00 and 20:00. Carrier on-time arrival for inbound is 81%. The operations manager wants to add doors, but capital approval will take two quarters.
1. Diagnose whether the dwell and departure problems are primarily a capacity (door-hours) problem or a flow-scheduling problem, and justify your conclusion with the numbers given.
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Door-hours available: 18 doors × 20 operating hours = 360 door-hours/day, minus handover and switching losses. Serving 270 total trailers means an average of 45 minutes of door occupancy per trailer is sustainable (360 × 60 / 270 = 80 minutes actually, before losses). So raw door-hours are not obviously scarce.
The real signal is temporal concentration. Inbound arrivals cluster 06:00–10:00 (55% of 140 ≈ 77 trailers in 4 hours ≈ 19 trailers/hour), while outbound departs 14:00–20:00. That means the dock must absorb a 4-hour inbound surge that is roughly 2.4× the daily average rate, then flip capacity to outbound later.
Conclusion: the symptoms are consistent with a flow-scheduling problem amplified by door-switching friction, not a pure door-count shortage. Evidence: (a) total door-hours appear adequate on average; (b) dwell is worst exactly when arrivals spike; (c) outbound misses rise because inbound trailers occupy doors into the afternoon, forcing 25-minute switches.
Recommendation before capex: smooth inbound appointments (level-loading, appointment windows with penalties), pre-assign doors by flow to minimize switches, and stagger outbound cutoffs. Adding doors would add capacity at the wrong time of day and would not fix the 06:00–10:00 surge.
2. The carrier on-time arrival rate is 81%. Explain how this interacts with the door-switching constraint and propose a sequencing rule that reduces missed outbound departures.
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Interaction: with 19% of inbound trailers arriving late, the dock cannot rely on scheduled arrival order. Late arrivals collide with the afternoon outbound push, and each unplanned door reassignment may trigger a 25-minute switch. The result is that outbound loads wait on doors that are occupied by trailers that were supposed to be gone.
Sequencing rule (priority-based door allocation):
- Protect outbound cutoffs first. Reserve a fixed block of doors (e.g., 8 of 18) as outbound-only during 14:00–20:00. Do not switch these doors for inbound.
- Rank inbound trailers by outbound dependency, not arrival time. A trailer whose freight feeds a 14:00 outbound departure gets priority over one feeding a 20:00 departure. Use a critical-ratio or slack-time rule: priority = (outbound cutoff − now) − expected unload duration.
- Buffer the surge. Schedule only ~85% of the 06:00–10:00 surge against door capacity, leaving slack for the 19% late arrivals; push the remainder into 10:00–12:00.
- Batch switches. Group door conversions into two fixed windows (e.g., 12:00 and 20:00) rather than ad hoc, capping switch losses.
This converts an unpredictable arrival stream into a protected outbound pipeline and cuts switch-induced delay.
3. Quantify the door-switching cost if the dock averages 40 door conversions per day, and assess whether eliminating half of them is a meaningful lever relative to the 22% outbound miss rate.
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Switching cost: 40 conversions × 25 minutes = 1,000 door-minutes/day = 16.7 door-hours/day. Against 360 gross door-hours, that is about 4.6% of total door capacity consumed by switching alone. Eliminating half saves ~8.3 door-hours/day.
Is it meaningful? On its own, 4.6% looks small, but the impact is concentrated in the afternoon outbound window where switches compete directly with departures. If most of the 40 conversions fall between 12:00 and 18:00, they consume a disproportionate share of the constrained period. Converting 8 doors to fixed outbound-only for that window removes roughly 20 conversions (half) and frees ~8 door-hours exactly when outbound demand peaks.
Judgment: switching reduction is a real but secondary lever. It should be paired with appointment level-loading. Alone, it likely moves the 22% miss rate by only a few points; combined with smoothing the 06:00–10:00 surge, it addresses the root cause. Frame it as a low-cost, fast-payback action while capex is pending, not a standalone fix.