Why Do We Have Speed Limits – and How Are They Set?

When DVSA remind us that “a speed limit is a limit, not a target,” the comments always light up. Some people argue that driving too slowly is more dangerous than driving too fast. Others question why limits exist at all. To make sense of these views, it helps to look at how limits have been set over time.

A short history of speed limits in the UK

• 1865 – The “Red Flag Act”: Early motor cars were restricted to 2 mph in towns and 4 mph elsewhere, with a man walking ahead carrying a red flag.

• 1934 – The 30 mph urban limit: As motor traffic boomed, Parliament introduced a blanket 30 mph limit in built-up areas. That core rule still applies today.

• 1970s – The 70 mph motorway limit: Initially a trial after a spate of high-speed crashes, the motorway ceiling was made permanent in 1967.

Over the decades, new categories have been added (20 mph zones, 50 mph for HGVs, variable motorway limits), but the principle has stayed the same: limits are a legal maximum, not a suggested speed.

The 85th percentile principle

From the mid-20th century onwards, engineers often used the 85th percentile to set limits. Here’s how it worked:

• Traffic surveys recorded the speed of free-flowing vehicles.

• The speed that 85% of drivers did not exceed was taken as the “safe” limit.

• The thinking was that most drivers behave reasonably, and the outliers at each end (the very slow and the very fast) cause most problems.

This was simple, data-driven, and often worked well on rural A-roads and motorways.

Why things have changed

Modern road safety thinking has moved away from the 85th percentile. Why?

• Vehicles are faster and quieter: Comfort in the cabin doesn’t reflect real risk outside.

• More vulnerable road users: Pedestrians, cyclists, mobility-scooter users – their safety isn’t reflected in “what drivers feel comfortable at.”

• Physics doesn’t compromise: At 20 mph a pedestrian has a strong chance of survival if struck; at 40 mph, the odds are grim.

That’s why we now use the Safe System approach (sometimes called “Vision Zero”): roads are designed so that human error doesn’t result in death or serious injury. Speed limits are set according to impact survivability, not just driver comfort.

Why the debate never ends

The psychology is simple:

• Drivers above the 85th percentile see cautious drivers as obstacles.

• Drivers below the 15th percentile see the faster ones as reckless.

• The majority in the middle adapt quietly, but the extremes argue the loudest.

This explains why DVSA’s “limit not target” message seems obvious to professionals but sparks outrage online. People tend to measure “safe speed” against their own comfort zone, not the needs of all road users.

The bottom line

Speed limits aren’t about punishing drivers or setting arbitrary rules. They’re about equalising expectations in a shared space. They create a baseline so that vulnerable road users, cautious drivers, and confident drivers can all use the same roads with fewer surprises.

A limit is the ceiling, not the target. The safe speed will often be less.