Transportation Logistics & Traffic Systems

# Transportation Logistics & Traffic Systems Knowledge Pack

## Supply Chain Logistics

### Freight Transportation Modes
Each mode has distinct cost, speed, and capacity characteristics that determine optimal use:

**Trucking (road)**: Most flexible mode — door-to-door service, no transshipment needed. FTL (full truckload, 40,000-45,000 lbs) costs $1.50-3.00/mile, transit time 500-600 miles/day. LTL (less than truckload, 150-15,000 lbs) consolidates multiple shippers onto one truck, costs more per pound but less total for smaller shipments. LTL typically adds 1-3 days for terminal handling and sorting.

Driver hours-of-service regulations limit driving to 11 hours within a 14-hour window after 10 consecutive hours off. Electronic logging devices (ELDs) enforce compliance. Team driving (two drivers alternating) enables near-continuous movement for time-sensitive freight.

**Rail**: Lowest cost for heavy, bulk freight over long distances. Intermodal (containers on flatcars) competes with trucking for distances >500 miles. Cost advantage: one train replaces 280+ trucks, consuming 1 gallon of fuel per ton per 470 miles vs. 130 miles for trucks. Disadvantage: slower (25-35 mph average including dwell time), less reliable schedules, requires truck drayage at each end.

**Ocean**: Moves 90% of global trade by volume. Container ships carry 10,000-24,000 TEU. Transit times: Trans-Pacific (Asia-West Coast) 12-18 days, Asia-East Coast via Suez 28-35 days. Cost per TEU: $1,000-5,000 depending on trade lane and market conditions. Vessel sharing agreements and alliances (2M, Ocean Alliance, THE Alliance) dominate major lanes.

**Air**: Fastest but most expensive ($4-8/kg vs. $0.10-0.30/kg ocean). Used for high-value, time-sensitive, or perishable goods. Represents <1% of global freight by weight but ~35% by value. Belly cargo (passenger aircraft) supplements dedicated freighters. Express integrators (FedEx, UPS, DHL) operate hub-and-spoke networks with guaranteed transit times.

### Warehouse Operations
The WMS orchestrates all warehouse activities: receiving (inbound inspection, putaway assignment), storage (slotting optimization — high-velocity SKUs in prime pick locations), picking (single order, batch, wave, or zone picking strategies), packing, and shipping.

**Slotting optimization**: Analyze SKU velocity (picks per period), cube (dimensions/weight), and affinity (frequently ordered together). Place fastest-moving SKUs in golden zone (waist to shoulder height, closest to shipping dock). Re-slot quarterly as demand patterns shift. Proper slotting reduces pick travel distance by 20-30%.

**Pick strategies**: Single-order picking is simplest but least efficient (one trip per order). Batch picking groups multiple orders, collecting all items in one pass. Wave picking releases orders in time-phased waves aligned with carrier pickup schedules. Zone picking assigns pickers to fixed zones; orders that span zones use conveyors or pass-through systems.

Labor is 50-65% of warehouse operating cost. Pick rate benchmarks: manual case picking 150-250 cases/hour, each picking 80-150 lines/hour, goods-to-person automation 300-600 lines/hour. ROI on automation depends on labor availability, volume consistency, and SKU characteristics.

### Vehicle Routing Problem
The VRP is the computational challenge of determining optimal routes for a fleet serving multiple customers. Basic VRP: minimize total distance/time for a fleet departing from a depot, visiting all customers once, and returning. Variants add real-world constraints:

**Capacitated VRP (CVRP)**: Vehicle capacity limits (weight, volume, pallets). Each route's total demand must not exceed vehicle capacity.

**VRP with Time Windows (VRPTW)**: Customers specify delivery windows. Late arrival = missed window = failed delivery attempt = costly re-delivery. Tight time windows dramatically reduce routing flexibility and increase fleet size requirements.

**Dynamic VRP**: Real-time order arrivals, traffic conditions, and vehicle breakdowns require continuous re-optimization. Modern TMS platforms use rolling-horizon optimization: plan routes based on known orders, re-optimize every 15-30 minutes as new orders and conditions arrive.

Exact solutions (optimal) are computationally intractable for >25-30 stops. Metaheuristics (genetic algorithms, simulated annealing, adaptive large neighborhood search) find near-optimal solutions (within 1-3% of optimal) for problems with hundreds of stops in seconds.

## Traffic Engineering

### Traffic Flow Theory
Fundamental relationship: Flow (q) = Density (k) × Speed (u). Flow is measured in vehicles per hour, density in vehicles per mile, speed in miles per hour.

The speed-density relationship is roughly linear: as density increases, speed decreases. At zero density, speed equals free-flow speed. At jam density (bumper-to-bumper), speed equals zero. Flow reaches maximum (capacity) at the optimal density point — approximately 40-50 vehicles/mile/lane for freeways.

The LOS scale (A through F) grades traffic operations. LOS A = free flow, LOS B = reasonably free flow, LOS C = stable flow with some restrictions, LOS D = approaching unstable flow, LOS E = at capacity, LOS F = forced/breakdown flow. Most highway agencies design for LOS C or D in urban areas.

### Highway Capacity Analysis
The HCM methodology determines capacity and LOS for highways, intersections, and other facilities.

**Freeway segments**: Base capacity is 2,400 passenger cars per hour per lane (pc/h/ln) under ideal conditions (12-ft lanes, adequate shoulders, flat terrain, all passenger cars). Adjustments for: lane width (narrow lanes reduce capacity), lateral clearance, terrain (grades — trucks on 4%+ grades create significant impact, adjusted via PCE factors; one truck on a steep grade = 2-4 passenger cars), driver population (unfamiliar drivers reduce capacity 10-15%).

**Signalized intersections**: Saturation flow rate is the maximum flow that can pass through a lane group during green time — approximately 1,900 pc/h/ln under ideal conditions. Adjusted for lane width, heavy vehicles, grade, parking activity, bus blockage, area type, and turning movements. V/C ratio = (demand volume × cycle length) / (green time × saturation flow rate). V/C > 1.0 means demand exceeds capacity — queue growth is unbounded.

**Signal timing**: Cycle length, phase splits, and offsets determine intersection performance. Webster's formula gives minimum-delay optimal cycle length: Co = (1.5L + 5) / (1 - Y), where L = total lost time per cycle and Y = sum of critical phase flow ratios. Longer cycles increase capacity but also increase delay for side-street traffic. Coordinated signal systems (green waves) optimize offsets between adjacent intersections to minimize stops on arterials.

### Intelligent Transportation Systems
ITS uses technology to improve transportation safety, efficiency, and reliability.

**ATMS components**: Traffic sensors (loop detectors, radar, video), communication networks (fiber, cellular), traffic management center (TMC), dynamic message signs (DMS), ramp meters, and adaptive signal control.

**Adaptive signal control**: Traditional signal plans are time-of-day based — fixed plans for AM peak, PM peak, off-peak. Adaptive systems (SCOOT, SCATS, InSync, SynchroGreen) adjust signal timing in real-time based on detector data. Benefits: 10-25% reduction in delay and stops compared to fixed-time plans. Requires reliable detection at every approach — a failed detector degrades the entire system.

**Ramp metering**: Traffic signals on freeway on-ramps release vehicles at controlled rates to prevent mainline breakdown. Release rate set based on mainline occupancy detected by loop detectors. When mainline density approaches critical density, ramp meter rates decrease (longer red times). Effective ramp metering increases mainline throughput by 5-10% during congestion.

**Connected and automated vehicles (CAV)**: Vehicle-to-infrastructure (V2I) communication enables TSP for buses, emergency vehicle preemption, speed advisories for green wave arrival, and work zone warnings. Vehicle-to-vehicle (V2V) communication enables cooperative adaptive cruise control (platooning reduces headways from 2 seconds to 0.5 seconds, increasing capacity 100%+ theoretically).

## Fleet Management

### Telematics and Tracking
Modern fleet management relies on GPS tracking, engine diagnostics (OBD-II port), and cellular communication. Key data streams: real-time location (every 30-120 seconds), speed, harsh braking/acceleration events, idle time, fuel consumption, engine fault codes, and driver behavior scoring.

Geofencing triggers alerts and actions when vehicles enter or exit defined areas — arrival notification to customers, automatic timecard entries, unauthorized use detection. Integration with TMS enables automated ETA updates based on actual position and traffic conditions vs. planned routes.

### Fuel Management
Fuel is 25-35% of fleet operating cost. Reduction strategies: speed management (fuel consumption increases roughly with the square of speed — reducing highway speed from 75 to 65 mph saves 10-15% fuel), idle reduction (one hour of idling = 0.8-1.0 gallons consumed; automatic engine shutdown timers, auxiliary power units for sleeper cabs), tire pressure monitoring (underinflated tires increase rolling resistance — every 1 PSI below optimal costs 0.3% fuel efficiency), aerodynamic devices (trailer skirts, tail fairings, gap reducers save 5-12% combined).

Route optimization contributes significantly: right-turn routing (UPS famously avoids left turns to reduce idle time at intersections), avoiding congestion zones during peak hours, elevation-aware routing (minimize climbing for heavy loads).

### Maintenance Scheduling
Preventive maintenance intervals based on mileage, engine hours, or calendar time. Critical items: oil and filter changes (every 15,000-25,000 miles for heavy trucks with synthetic oil), tire inspections (every trip — pre-trip and post-trip DOT-required inspections), brake inspections (every 25,000 miles), and DOT annual inspections (mandatory, 21-point minimum).

Predictive maintenance uses telematics data to identify developing issues before failure. Engine fault codes, oil analysis trends (metal particles indicate bearing wear), coolant temperature trends, and DPF (diesel particulate filter) regeneration frequency all provide early warning. Unplanned breakdown costs 3-5x more than planned maintenance due to towing, expedited parts, driver downtime, and service failure penalties.

### Last-Mile Delivery
Last mile accounts for 40-50% of total delivery cost due to low density, failed delivery attempts, and urban congestion. Strategies to reduce cost:

**Delivery density optimization**: Offer customers incentive pricing for flexible delivery windows. Cluster deliveries geographically — Monday north side, Tuesday south side. Achieving 15+ stops per hour vs. 8 stops per hour cuts per-package cost nearly in half.

**Alternative delivery points**: Locker networks (Amazon, InPost), retail pickup (BOPIS — buy online, pick up in store), and attended delivery to neighbors/safe places reduce failed delivery attempts. Failed first-attempt rates of 5-15% create costly re-delivery cascades.

**Micro-fulfillment**: Position inventory in urban micro-warehouses or dark stores for same-day/next-day delivery within 2-5 mile radius. Reduces last-mile distance dramatically but adds real estate and inventory carrying costs. Economics work for high-velocity, high-margin SKUs in dense urban markets.