Motorists routinely overestimate how much time they save by driving faster, a psychological phenomenon researchers call the time-saving bias. The mathematics underlying journey duration shows that speed increases deliver progressively smaller benefits as velocity rises, while real-world conditions including traffic, junctions, and speed limit variations erode theoretical time savings to near-irrelevance on most trips. Meanwhile, crash risk and fuel consumption increase dramatically, creating dangerous trade-offs for marginal benefits drivers consistently overvalue.
The Mathematical Reality
Journey time equals distance divided by speed, a simple relationship that produces counterintuitive results. For a fixed distance, each incremental speed increase saves less time than the previous increment. Driving 100 kilometres at 100 km/h requires 60 minutes. Increasing speed to 110 km/h reduces journey time to approximately 54.5 minutes, saving just 5.5 minutes. That 10 km/h increase, representing 10 percent more speed, delivers only 9 percent time reduction.
The pattern intensifies as baseline speed rises. Increasing from 80 km/h to 90 km/h on that same 100-kilometre journey saves approximately 8.3 minutes, but increasing from 120 km/h to 130 km/h saves only 4.3 minutes despite identical 10 km/h increments. The faster you already travel, the less additional speed helps.
Research published in journals including Transportation Research demonstrates that people systematically misjudge these relationships. Drivers underestimate time saved when increasing already-low speeds and overestimate savings from increasing already-high speeds, precisely the opposite of mathematical reality. This bias encourages dangerous speeding where it provides minimal benefit while discouraging efficient driving at moderate speeds where time impacts prove larger.
Professor Ian Walker of Swansea University, who has researched driver speed perception, explained the phenomenon in a 2023 study published in Accident Analysis and Prevention: "Our brains process speed increases as linear time savings when the relationship is actually hyperbolic. A 10 mph increase feels like it should save the same time regardless of baseline speed, but mathematics says otherwise. This systematic error influences countless daily driving decisions with real safety consequences."
Real World Complications
The theoretical mathematics assume constant speeds, rarely achievable in actual driving. Traffic signals, congestion, roadworks, weather conditions, slower vehicles, and speed limit variations all constrain average speeds well below the maximum velocities drivers achieve on clear sections. New Zealand Transport Agency research examining actual journey times on state highways found that increasing maximum speed limits by 20 km/h only raised average speeds by approximately 8 to 12 percent because drivers spend substantial time below maximum limits.
Consider a 270-kilometre journey, approximately three hours at 90 km/h. Pure mathematics suggests that maintaining constant 130 km/h would reduce journey time to 2 hours 5 minutes, saving nearly an hour. However, real driving conditions make sustained 130 km/h impossible. Accounting for towns, traffic, curves, and slower sections that realistically constrain average speeds means the actual time saving typically shrinks to 20 to 30 minutes at best, often far less.
Studies of British motorway journeys conducted by road safety researchers found similar patterns. Drivers attempting to maintain 90 mph where possible versus those cruising at 70 mph on 150-mile trips saved an average of just 12 minutes, far below the 30-minute theoretical maximum, because actual average speeds differed by only 8 mph rather than the 20 mph maximum speed difference.
The Diminishing Returns Curve
The time-saving bias manifests clearly when examining specific speed ranges. Increasing from 40 mph to 50 mph on a 30-mile journey reduces travel time from 45 minutes to 36 minutes, a substantial 9-minute saving. The same 10 mph increase from 80 mph to 90 mph over identical distance cuts travel time from 22.5 minutes to 20 minutes, saving just 2.5 minutes.
This mathematical reality means that speeding delivers greatest time benefits precisely where speeds are already constrained by safety considerations, such as residential areas and urban roads where pedestrians, cyclists, and complex road layouts create genuine crash risks. The speeds where time savings become trivial, motorways and dual carriageways above 70 mph, are the contexts where drivers most commonly exceed limits seeking marginal gains.
Transport consultancy analysis for Britain's Department for Transport modeled journey time impacts across various scenarios. Their findings showed that for typical commuter journeys of 20 to 30 miles, the difference between strict speed limit compliance and consistent 10 mph speeding amounted to between 3 and 7 minutes depending on route characteristics. Driver surveys, however, found that respondents estimated time savings from speeding at 15 to 20 minutes for similar journeys, triple the actual benefit.
Risk Rises Exponentially
While time savings diminish with speed increases, crash risk and injury severity rise dramatically. Stopping distances increase exponentially with speed because kinetic energy grows as the square of velocity. A vehicle traveling at 70 mph carries twice the kinetic energy of the same vehicle at 50 mph, not 40 percent more as the speed increase might suggest.
According to research published by the Royal Society for the Prevention of Accidents, crash risk approximately doubles for every 5 mph above average traffic speed. A driver traveling 10 mph faster than surrounding traffic faces roughly four times the crash risk of someone matching flow speeds. Impact forces in crashes rise similarly, with pedestrian fatality risk increasing from approximately 10 percent at 20 mph impact speeds to over 80 percent at 40 mph according to Transport Research Laboratory collision studies.
The trade-off proves starkly unfavorable. Speeding by 10 mph on a typical motorway journey might save 5 minutes while doubling crash risk. On many urban journeys, where traffic signals and congestion limit average speeds regardless of brief high-speed bursts, actual time savings shrink to under 2 minutes while crash risks in residential areas where vulnerable road users are present rise substantially.
Fuel Consumption Compounds Costs
Aerodynamic drag increases with the square of velocity, meaning fuel consumption rises dramatically at higher speeds. Research by the AA and various automotive organizations shows that increasing motorway cruising speed from 70 mph to 85 mph typically increases fuel consumption by 20 to 25 percent while saving minimal journey time due to the diminishing returns already discussed.
For a typical three-hour motorway journey, maintaining 70 mph versus attempting 85 mph where possible might save 15 minutes but consume an additional 3 to 4 litres of fuel in a conventional car. At current British fuel prices around £1.50 per litre, that represents £4.50 to £6.00 additional cost, roughly £20 to £24 per hour when calculated against actual time saved. Few people would consider their time worth so little in other contexts, yet driving behavior suggests many assign it minimal value.
Why The Bias Persists
Several psychological factors sustain the time-saving bias despite mathematical reality. Speed provides visceral sensation and sense of progress that cruising at moderate speeds lacks. The feeling of overtaking slower vehicles and rushing scenery creates subjective experience of accomplishment disconnected from actual time savings.
Drivers also tend to remember the clear road sections where high speeds were possible while forgetting or discounting the constrained sections that determine average speeds. This selective memory reinforces beliefs that speeding delivers significant benefits because the high-speed portions remain psychologically salient while slower sections fade from recollection.
The delayed feedback between driving behavior and journey outcomes compounds the problem. Unlike situations where cause and effect connect immediately, drivers typically cannot accurately assess whether speeding saved meaningful time because they lack baseline comparison on specific journeys and the 3 to 5 minute differences that speeding typically produces are easily attributed to other factors like traffic variations.
Policy Implications
Understanding the time-saving bias suggests that traditional road safety messaging emphasizing danger may prove less effective than education about actual time costs and benefits. Campaigns in New Zealand and Australia have begun highlighting that "rushing saves minutes, not hours" to reframe speeding decisions around realistic benefit assessment rather than fear-based risk messaging alone.
Variable speed limits on British motorways attempt to optimize traffic flow by reducing speed variations between vehicles and preventing the stop-start waves that constrained average speeds create. These systems typically reduce maximum speeds during busy periods while paradoxically decreasing journey times by smoothing traffic flow, demonstrating that lower speeds can actually improve outcomes when congestion would otherwise force frequent braking.
The evidence collectively shows that for most journeys, speeding delivers marginal time savings, typically under 10 minutes and often under 5 minutes, while substantially increasing crash risk, fuel costs, and stress. The time-saving bias causes drivers to systematically overestimate benefits by factors of three to five, leading to poor risk-benefit decisions that endanger themselves and others for gains barely noticeable in daily schedules.
A calm 90 km/h versus an aggressive 130 km/h on a three-hour journey realistically differs by perhaps 20 to 30 minutes at best, often far less, while transforming a relaxed drive into a stressful, expensive, dangerous rush. The mathematics are clear, the real-world evidence overwhelming, yet the bias persists because our brains struggle with hyperbolic relationships and subjective speed sensations override rational calculation. Understanding this disconnect between perception and reality offers hope for better driving decisions, if drivers can overcome the deeply ingrained intuition that faster must mean significantly quicker. It doesn't, and the costs of pretending otherwise keep adding up in crashes, fuel bills, and stress levels that dwarf the few minutes occasionally saved.