IB Geography G.3.2 Traffic Congestion Patterns, Trends and Impacts Notes

Congestion Forms Where Travel Demand Exceeds Road Capacity

1. Traffic congestion: Road users slow down and journeys become less reliable when the demand for road space approaches or exceeds the network’s capacity.
2. Peak periods: Commuting and school journeys concentrate demand into short morning and evening periods.
3. Radial routes: Roads leading towards a central business district carry converging flows and often become bottlenecks.
4. Junctions: Signals, turning movements and merging traffic reduce the effective capacity of a road corridor.
5. Incidents: Crashes, roadworks and breakdowns can cause sudden congestion even when normal demand is manageable.
6. Last-mile concentration: Deliveries, taxis and ride-hailing vehicles add stopping and turning movements in commercial districts.

Traffic Flow Breaks Down Before a Road Is Physically Full

1. Flow: The number of vehicles passing a point per unit of time increases as traffic becomes denser until the road reaches maximum throughput.
2. Speed: Drivers reduce speed as the gap between vehicles narrows and manoeuvres become harder.
3. Density: Vehicles per kilometre rise as traffic accumulates on a road segment.
4. Unstable flow: Near capacity, a small brake or lane change can create a shock wave that travels backward through traffic.
5. Queue spillback: A queue from one junction can block an upstream junction and spread delay across connected streets.
6. Effective capacity: Bus stops, loading, parking, turning traffic and roadworks reduce the moving space available without changing the road width.
7. Reliability: Travellers often allow time for a bad day rather than the average day, so variable journeys impose a cost even when mean speed changes little.
8. Network effect: A blockage on one link diverts traffic to alternatives and can overload routes that were not initially congested.

Land Use and Income Reshape Congestion over Time

1. Urban sprawl: Low-density development lengthens journeys and makes frequent public transport harder to provide.
2. Rising incomes: Higher household incomes can increase car ownership when road travel is convenient and parking is available.
3. Jobs-housing separation: Congestion increases when affordable housing and major employment centres are far apart.
4. Public transport quality: Reliable and affordable alternatives reduce the need to travel by private car.

1. Induced demand: Extra road capacity can generate additional traffic as routes, travel times and locations adjust.
2. Digital change: Remote work can reduce some commuter trips while online shopping increases delivery traffic.

Congestion Has Distinct Daily, Weekly and Spatial Patterns

1. Morning peak: Home-to-work and school journeys converge within a narrow time window.
2. Evening peak: Trips are often more dispersed because shopping, leisure and varied work finishing times add destinations.
3. Radial pattern: Routes towards a dominant centre carry directional peaks when jobs remain concentrated in the CBD.
4. Orbital pattern: Ring roads and cross-town routes congest where dispersed suburbs depend on a few river crossings or motorway junctions.
5. School-term effect: Traffic can fall during school holidays because school trips and their interaction with commuting disappear.
6. Weekend effect: Retail and leisure districts may replace commuter corridors as the main hot spots.
7. Freight timing: Port, warehouse and market schedules can create peaks outside the standard commuting periods.
8. Polycentric city: Several employment centres produce intersecting flows rather than one simple movement towards downtown.

Travel Behaviour Adjusts to New Capacity and New Prices

1. Route response: Drivers move from slower roads to a newly widened route.
2. Time response: Some travellers shift into the peak once the journey initially becomes faster.
3. Mode response: A faster road can attract trips that were previously made by rail, bus, walking or cycling.
4. Destination response: Households and firms may choose more distant locations when road access improves.
5. Suppressed demand: A congested network hides trips that people would make if the cost in time fell.
6. Induced demand: These adjustments use part of the added capacity and can restore congestion over time.
7. Feedback: More traffic can weaken bus reliability, which makes private driving attractive to travellers who have access to a car.

Congestion Transfers Costs Across the City

1. Time cost: Unreliable journeys reduce productive time and make work, education and appointments harder to reach.
2. Operating cost: Stop-start traffic increases fuel or electricity use and raises delivery costs.
3. Air pollution: Exhaust emissions and brake and tyre wear concentrate along busy corridors.
4. Noise: Engines, horns and repeated acceleration reduce environmental quality near major roads.
5. Emergency access: Slow traffic can delay ambulances, fire services and buses.
6. Unequal impact: People without cars may still bear pollution and bus delays created by road traffic.
7. Business trade-off: Firms benefit from access to customers and labour but lose money when deliveries become slow and unpredictable.

Congestion Creates External Costs That Market Prices Do Not Capture

1. Private cost: A driver bears fuel, vehicle wear, fares or parking charges and part of the journey time.
2. Delay externality: One additional vehicle slightly slows many other road users near capacity.
3. Pollution externality: Residents beside the corridor receive emissions even when they do not make the trip.
4. Noise externality: Acceleration, braking and horns reduce sleep and amenity near busy routes.
5. Bus penalty: A full bus can carry many people but suffers delay created by lower-occupancy vehicles unless it has priority.
6. Freight penalty: Unreliable arrival times require firms to hold more stock or schedule spare vehicle and staff time.
7. Emergency penalty: A delayed ambulance imposes a cost that is not reflected in the price of the journey causing the delay.
8. Spatial inequality: Low-income households often live near arterial roads or rely on buses, so they can bear costs without owning a car.
9. Agglomeration limit: Long and unreliable journeys shrink the practical labour market that firms and workers can reach.

Management Changes Price, Capacity and Travel Choices

1. Road pricing: Charges make drivers face part of the external cost imposed on other road users.
2. Parking policy: Higher prices and lower parking supply reduce the convenience of driving to congested centres.
3. Public transport: Frequent buses and rail services provide an alternative that can move more people through limited street space.
4. Bus priority: Dedicated lanes and signal priority make bus journey times more reliable.
5. Active travel: Safe walking and cycling networks can replace short car journeys.
6. Traffic management: Coordinated signals, loading controls and incident response improve the use of existing capacity.
7. Equity safeguard: Discounts, exemptions and investment in public transport can reduce the burden on groups with few travel alternatives.

Each Strategy Targets a Different Part of the Congestion System

1. Cordoned charge: Drivers pay to cross a boundary around a congested district.
2. Area charge: A vehicle is charged for travelling within a defined zone during specified hours.
3. Distance charge: Payment rises with the distance travelled and can vary by place, time or vehicle type.
4. Parking control: Limited supply and higher prices raise the cost of ending a car journey in the centre.
5. Bus priority: Reserved lanes and signal priority protect high-capacity vehicles from general traffic delay.
6. Road capacity: New lanes or junction changes can remove a bottleneck but may move the queue to the next constraint.
7. Travel planning: Employers and schools can stagger schedules, support shared travel or reduce trips at the peak.
8. Transit-oriented development: Housing and jobs placed around frequent public transport reduce the need for long car journeys.
9. Freight management: Consolidation centres and timed deliveries reduce repeated van trips and kerbside conflict.
10. Information: Real-time routing helps individual drivers but can transfer traffic into residential streets.

A Fair Package Links Restraint to a Usable Alternative

1. Effectiveness: Measure person throughput and journey reliability as well as vehicle speed.
2. Equity: Compare costs for income groups, disabled travellers, shift workers and areas with weak public transport.
3. Spatial leakage: Check boundary roads and uncharged districts for diverted traffic.
4. Temporal leakage: Check whether traffic moves outside charging hours rather than disappearing.
5. Revenue use: Investment in buses, rail, walking and cycling can widen the alternatives available to charged drivers.
6. Durability: Repeat measurements after travellers, firms and land use have had time to adjust.

London Reduced Central Traffic but Congestion Later Returned

1. Context: Central London has a dense concentration of jobs and visitors within a street pattern that cannot easily be widened.
2. Congestion Charge: London introduced a daily charge for driving in the central zone in 2003.
3. Early traffic effect: Transport for London reported that traffic in the original charging zone was 21 per cent below pre-charge levels after five years.
4. Public transport: Charge revenue and wider transport policy supported buses and alternatives to private car travel.
5. Road-space change: More space was allocated to pedestrians, cyclists, buses and street works, which reduced the capacity available to general traffic.
6. Congestion outcome: Transport for London reported that congestion had returned to pre-charge levels by 2008 despite lower traffic volumes.

London Shows Why Traffic Volume and Congestion Must Be Measured Separately

1. Traffic entering the zone: Transport for London reported a 21 per cent fall after five years compared with pre-charge levels.
2. Daily vehicles: That reduction was about 70,000 fewer cars entering the original zone each day.
3. Street capacity: Utility works and changes that supported pedestrians and other road users reduced space for general traffic.
4. Congestion: Delay therefore returned to pre-charge levels even though fewer vehicles entered the zone.
5. Counterfactual: The relevant comparison is not only London before and after charging but what traffic would have been without the charge.
6. Boundary: Monitoring must include the Inner Ring Road and nearby districts where drivers may divert.
7. Time: An early improvement can weaken as travel behaviour, construction and street allocation change.

London Combines Charging with Cleaner Vehicles and Modal Shift

1. ULEZ: The Ultra Low Emission Zone began in central London in 2019 and expanded London-wide in 2023.
2. Different objective: ULEZ charges vehicles that do not meet emission standards, so its main purpose is cleaner air rather than reduced congestion.
3. Network measures: Bus priority, rail investment and segregated cycle routes give some travellers alternatives to driving.
4. Strength: Pricing can reduce traffic entering a defined area without the land and construction required for major new roads.
5. Limitation: Drivers with poor public transport access may face a higher relative burden than residents with several alternatives.
6. Boundary effect: Traffic may divert around a charged area, so monitoring must include boundary roads as well as the zone itself.
7. Judgement: London demonstrates that pricing works best as part of a wider package and must be evaluated with separate measures of traffic, speed, emissions and accessibility.

London's Measures Have Different Objectives and Affected Populations

1. Congestion Charge: The charge targets where and when vehicles travel in central London.
2. Ultra Low Emission Zone: The emissions standard targets the cleanliness of a vehicle rather than the amount of road space it occupies.
3. Bus lane: Priority can improve person throughput even when the general traffic lane becomes slower.
4. Cycle route: Protected space can replace short car trips but may create a short-term capacity dispute during construction.
5. Exemptions: Rules for residents, disabled people and specialist vehicles alter both equity and traffic response.
6. Revenue: By law, net revenue from the charge supports transport, linking restraint to alternatives.
7. Political power: The Mayor and Transport for London set charging rules, while boroughs control many surrounding streets.
8. Business response: Firms can reschedule deliveries, consolidate loads or pass part of the charge to customers.
9. Household response: A commuter with rail access has more ability to change mode than a night worker travelling across poorly connected suburbs.
10. Final judgement: London reduced central traffic through pricing, but lasting congestion relief depends on road-space management and attractive alternatives.