FOLLOWING DISTANCE GUIDE
The science behind NZ's 2-second and 4-second rules. Why time-based gaps work better than fixed distances, and how to stay safe on New Zealand roads.
NZ USES THE 2/4-SECOND SYSTEM
New Zealand officially uses a 2-second rule for dry conditions and 4-second rule for wet, slippery, or towing situations. This is different from the 3-second rule used in the USA.
LEGAL ≠ SAFE
NZ has enforceable legal minimum distances (e.g., 36m at 90+ km/h), but these are ~1.4 seconds — less than the recommended 2-second rule. Legal minimum is not the safe minimum.
TIME SCALES WITH SPEED
A time-based gap automatically adjusts with speed. At 50 km/h, 2 seconds = 28m. At 100 km/h, 2 seconds = 56m. No mental maths required.
🇳🇿 NZ OFFICIAL RULES (NZTA ROAD CODE)
The NZTA Road Code is New Zealand's official guide to road rules. It teaches a time-gap system that automatically scales with speed, rather than fixed metre distances which are hard to judge while driving.
📋 How to Use the Rule (NZTA Method)
1. Watch the vehicle ahead pass a landmark (sign, tree, power pole)
2. Start counting: "one thousand and one, one thousand and two..."
3. If you pass the landmark before finishing, you're too close — slow down
4. Double the count (to 4 seconds) in wet weather or when towing
⚖️ NZ LEGAL MINIMUM DISTANCES
Under the Land Transport (Road User) Rule 2004, Section 5.9, New Zealand has enforceable legal minimum following distances. These are below the recommended 2-second rule and represent the absolute minimum, not a safe target.
| Speed | Legal Minimum | Time Equivalent | 2-Second Rule | Shortfall |
|---|---|---|---|---|
| 40-50 km/h | 16 metres | ~1.3 sec | 28m | -12m |
| 50-60 km/h | 20 metres | ~1.3 sec | 33m | -13m |
| 60-70 km/h | 24 metres | ~1.4 sec | 39m | -15m |
| 70-80 km/h | 28 metres | ~1.4 sec | 44m | -16m |
| 80-90 km/h | 32 metres | ~1.4 sec | 50m | -18m |
| 90+ km/h | 36 metres | ~1.4 sec | 56m | -20m |
⚠️ Key Takeaway: Legal ≠ Safe
The legal minimum (~1.3–1.5 seconds) provides no buffer for unexpected events. It assumes perfect attention, instant reaction, and ideal braking conditions. The 2-second rule provides the margin needed for real-world driving. You can technically be fined at ~1.5 seconds, but the Road Code recommends 2 seconds as safe practice.
NUMERICAL PROOF: WHY LEGAL MINIMUMS ARE INADEQUATE
At 100 km/h, here's what different gaps actually provide:
| Following Gap | Distance | Assessment |
|---|---|---|
| Legal minimum | 36m (~1.3s) | ❌ No reaction buffer — assumes instant response |
| 2-second rule | 55.6m | ✓ Minimal safe gap for alert drivers, dry roads |
| 3-second rule | 83.3m | Better margin — used in USA/Canada as standard |
| 4-second rule | 111.1m | ✓ Required for wet/adverse — NZ recommended |
🧮 FOLLOWING DISTANCE CALCULATOR
Calculate Your Following Distance
Recommended following distance based on your inputs
⚡ BRAKE REACTION TEST
🔴🔴 Both brake lights come on
🟡 Turn signals are traps!
| Reaction Time | Rating | Real-World Context |
|---|---|---|
| < 500ms | 🏆 Exceptional | Highly alert, anticipating hazards |
| 500–700ms | ⚡ Excellent | Alert driver, good conditions |
| 700–900ms | ✓ Good | Normal attentive driving |
| 900–1200ms | ⚠️ Average | Slightly distracted or fatigued |
| > 1200ms | ⛔ Slow | Fatigued, distracted, or impaired |
📊 Why This Test Works
Unlike simple "click when green" tests (~200ms), this measures recognition + decision + response. You must identify brake lights vs turn signals, decide whether to react, then tap. Results of 600-1000ms are realistic for attentive driving.
🧠 THE SCIENCE: PERCEPTION-REACTION TIME
Stopping distance isn't just about brakes — it's about the time your brain takes to perceive a hazard, decide what to do, and react by moving your foot to the brake. During this time, your car is still moving at full speed.
What Happens in an Emergency Stop
0.75–1.5s
0.5–1.0s
Variable
REACTION TIME BY DRIVER STATE
| Driver State | Typical Reaction Time | Distance at 100 km/h | Notes |
|---|---|---|---|
| Alert, expecting stop | 0.6–0.7 sec | 17–19m | Test conditions, hand ready |
| Normal driving | 1.0–1.5 sec | 28–42m | Typical attentive driver |
| Elderly / Fatigued | 2.0–2.5 sec | 56–69m | Reduced cognitive speed |
| AASHTO Design Standard | 2.5 sec | 69m | 90th percentile coverage |
📚 Why AASHTO Uses 2.5 Seconds
The American Association of State Highway and Transportation Officials (AASHTO) uses 2.5 seconds for road design because it covers 90% of all drivers in simple to moderately complex situations. This is the foundation of the 3-second rule used in the USA — adding buffer to 2.5 seconds of reaction time.
THE STOPPING DISTANCE EQUATION
Total stopping distance combines reaction distance (distance travelled while your brain processes) and braking distance (distance to physically stop):
Where: v = speed (m/s), t = reaction time (seconds), a = deceleration (m/s²)
This is why time-based following gaps work so well — they automatically account for the v × t component scaling with speed. The 2-second and 4-second rules approximate this physics in a form drivers can actually use.
WHY TIME RULES SCALE AUTOMATICALLY
The brilliance of the 2/4/6-second system:
| Rule | What It Covers |
|---|---|
| 2-second rule | ~1.0–1.5s reaction time + some braking margin in good conditions |
| 4-second rule | Buffer for reduced friction (wet μ), longer reaction times, reduced visibility |
| 6-second rule | Heavy mass, brake lag, reduced deceleration capability of trucks |
⏱️ WHY TIME WORKS BETTER THAN METRES
The brilliance of time-based rules is that they automatically scale with speed. You don't need to calculate metres while driving — just count seconds.
DISTANCE COVERED IN 2 SECONDS
| Speed | 2 Seconds | 3 Seconds | 4 Seconds |
|---|---|---|---|
| 30 km/h | 17m | 25m | 33m |
| 50 km/h | 28m | 42m | 56m |
| 80 km/h | 44m | 67m | 89m |
| 100 km/h | 56m | 83m | 111m |
| 110 km/h | 61m | 92m | 122m |
Formula: Distance (m) = Speed (km/h) ÷ 3.6 × Time (seconds)
🌏 HOW NZ COMPARES GLOBALLY
Different countries use different standard rules. NZ's 2/4-second system is similar to the UK, while the USA uses a more conservative 3-second baseline.
🤔 Why the Difference?
The USA builds directly on the AASHTO 2.5-second reaction time standard, then adds margin → 3 seconds as a single conservative number. NZ/UK prefer to separate conditions explicitly (2s dry, 4s wet) rather than one blanket rule. This is policy choice, not physics disagreement.
📊 THE COST OF TAILGATING
Rear-end collisions are one of the most common crash types globally, and tailgating is a leading cause.
~30% OF ALL CRASHES
Rear-end collisions account for approximately 30% of all motor vehicle crashes according to NHTSA data.
50-89% INVOLVE TAILGATING
Research indicates between 50% and 89% of rear-end collisions are either caused by or involve tailgating behaviour.
1 IN 8 UK MOTORWAY CRASHES
Tailgating is a factor in approximately 1 in 8 crashes on England's motorways and major A roads.
🚨 What Tailgating Eliminates
Time to perceive: You can't see past the vehicle in front
Time to react: No buffer between seeing danger and needing to stop
Space to stop: Even perfect braking won't help if there's no room
⚡ SPECIAL SITUATIONS
WHEN TO INCREASE FOLLOWING DISTANCE
| Situation | Minimum Gap | Why |
|---|---|---|
| 🌧️ Wet roads | 4 seconds | Braking distance doubles; reduced tyre grip |
| ❄️ Ice or snow | 10× normal | Braking distance can be 10× longer on ice |
| 🚐 Towing trailer | 4+ seconds | Increased mass; trailer sway risk |
| 🚛 Following trucks | 4+ seconds | Can't see ahead; debris risk; sudden stops |
| 🌙 Night driving | 3+ seconds | Reduced visibility; slower hazard perception |
| 😴 Fatigued | 4+ seconds | Reaction time significantly increased |
| 🔋 Electric vehicles | 3 seconds | Heavier due to batteries; some experts recommend longer gaps even in dry conditions |
🔋 Special Note: Electric Vehicles
EVs are typically 20-30% heavier than equivalent petrol vehicles due to battery weight. While regenerative braking can help in some situations, the increased mass means longer stopping distances at highway speeds. Some safety experts recommend EVs use a 3-second rule even in dry conditions.
🚗 Being Tailgated?
Don't: Brake-check, speed up, or engage the driver
Do: Gradually slow down, move over when safe, let them pass. If they continue following, pull over completely and let them go.
🛞 17 FACTORS THAT AFFECT YOUR STOPPING DISTANCE
Following distance rules are guidelines — your actual stopping distance depends on many variables. Our Braking Simulator models all 17 of these factors using physics validated against real tyre tests.
🚨 THE BIG FIVE (MOST IMPACT)
| Factor | Impact Range | Key Insight |
|---|---|---|
| 1. Road Surface | 0.03–0.90 μ | Wet ice (μ=0.03) vs rough dry asphalt (μ=0.90) = 30× difference |
| 2. Water Depth | −55% grip | Damp (0.1mm) = 10% loss. Heavy rain (1.5mm) = 40% loss. Flooded = 55%+ loss |
| 3. Tread Depth | +44% braking | 1.6mm vs 8mm tread = 44% longer wet braking (Continental test data) |
| 4. EU Wet Grip Grade | ±15% grip | Grade A = 1.15× baseline. Grade E = 0.80× baseline. One grade = ~5% |
| 5. Speed | v² scaling | Double speed = 4× braking distance (physics: d = v²/2a) |
⚙️ TYRE CONDITION FACTORS
🌡️ ENVIRONMENTAL FACTORS
🚗 VEHICLE & SYSTEM FACTORS
| Factor | Effect | Notes |
|---|---|---|
| ABS System | +15-20% grip | Uses peak friction (0.80μ) instead of locked wheel sliding friction (0.65μ) |
| Vehicle Load | −5% per 500kg | Heavier vehicle = longer stopping. Tyre load sensitivity applies. |
| Brake Fade | −30% severe | Repeated hard braking heats brakes, reducing effectiveness. Mountain descents. |
| Vehicle Era | 1970s = 0.66μ | 1970s tyres/brakes: μ≈0.66. 1990s: μ≈0.85. 2020s: μ≈0.95+ |
| Downforce (Sports cars) | +10% @ 200km/h | Aerodynamic downforce increases tyre load at high speed |
💧 HYDROPLANING: THE SPEED KILLER
When water depth exceeds ~2.5mm (standing water/puddles), hydroplaning becomes a real risk. The NASA hydroplaning formula calculates the speed at which your tyres lose contact with the road:
Source: NASA Technical Note TN D-2056
| Factor | Effect on Hydroplaning |
|---|---|
| Tyre Pressure | Higher PSI = higher hydroplane speed (better). 32 PSI → ~105 km/h threshold |
| Tread Depth | More tread = more water evacuation. 8mm can evacuate 30+ litres/second |
| Tyre Width | Narrower = better in deep water (cuts through). 205mm vs 275mm matters |
| Water Depth | <2.5mm: no hydroplane risk. 2.5-5mm: moderate. >5mm: severe |
⚠️ The Tread Depth Cliff Effect
Wet grip doesn't decline linearly with tread depth — there's a "cliff" below 4mm where water evacuation collapses rapidly. At highway speeds with 1.6mm tread in heavy rain, you may already be hydroplaning before you know it. This is why safety experts recommend replacing tyres at 3mm, not the 1.5mm legal minimum.
🎮 SEE IT IN ACTION
Our physics-based braking simulator lets you experiment with all 17 factors. See exactly how your tyres, conditions, and driving affect stopping distance.
🚗 LAUNCH BRAKING SIMULATOR →Validated against Continental, ADAC, and TireRack test data • 285 real tyre tests • GPT-4 code reviewed
🔬 THE PHYSICS: IS THIS ACCURATE?
A common question: "How accurate is following-distance guidance?" Here's an honest assessment from a physics and engineering perspective.
WHAT THE SCIENCE GETS RIGHT
| Element | Status | Notes |
|---|---|---|
| Basic stopping equations | ✓ Correct | d = v×t + v²/2a is textbook physics |
| Time-based scaling | ✓ Correct | Automatically adjusts for speed |
| Wet vs dry separation | ✓ Correct | Reflects real friction differences |
| Heavy vehicle allowance | ✓ Correct | Mass affects stopping distance |
WHERE REAL-WORLD ACCURACY IS LIMITED
No road-safety guidance can be "100% accurate" because reality involves variables that can't be pre-calculated:
📚 Academic Perspective
A transport engineering professor would classify the 2/4/6-second system as a validated educational model — it uses correct governing equations, dimensionally consistent physics, and plausible real-world coefficients. However, it's deterministic guidance for a statistical reality. This is why it's a rule of thumb, not a guarantee.
Bottom line: The physics underlying these rules is sound. The rules are conservative approximations designed to keep most drivers safe in most conditions — which is exactly what good safety guidance should be.
✓ SUMMARY: WHAT TO REMEMBER
| Normal dry conditions | 2 SECONDS |
| Wet, slippery, frost, towing | 4 SECONDS |
| Heavy vehicles, truck-trailers | 6 SECONDS |
| Ice or snow | 10× NORMAL |
💡 The Simple Test
Pick a landmark. When the car ahead passes it, count: "one thousand and one, one thousand and two". If you pass the landmark before you finish, you're too close. Double the count in wet weather.
