A red light is showing, yet a green arrow appears beside it. For many learners, that moment can feel confusing – especially when traffic is waiting behind you. So, what is a filter light on a traffic light in the UK? It is a directional signal, usually a green arrow, which allows traffic to move in the direction shown even when the main signal is red.

Filter lights are designed to keep junctions moving safely and efficiently. However, they are not a reason to rush, assume the road is clear or follow the car in front without checking. Understanding exactly what the arrow permits will help you make calm, safe decisions on lessons and during your driving test.

What is a filter light on a traffic light in the UK?

A filter light is commonly called a filter arrow or green filter arrow. It is a green arrow displayed next to, above or within a set of traffic lights. The arrow points in the direction that traffic is permitted to travel.

For example, you may be waiting at a red traffic light to turn right. If a green right-pointing arrow illuminates, you can make that right turn, provided it is safe to do so. The red circular light still applies to other movements, such as going straight ahead or turning left, unless another signal tells you otherwise.

The word ‘filter’ describes the way the signal separates traffic movements. Instead of every vehicle moving at once, the junction gives one stream of traffic a controlled opportunity to move. This can reduce conflict with oncoming vehicles and help prevent queues building up.

What a green filter arrow allows you to do

A green filter arrow gives permission to proceed only in the direction of that arrow. A left arrow means you may turn left. A right arrow means you may turn right. At some larger junctions, there may be a straight-ahead arrow, or more than one arrow for different lanes.

The key word is ‘only’. If a right-hand filter arrow is showing, it does not allow you to drive straight on. If the lane markings require a left turn and the left filter arrow is lit, you should follow that direction.

You may pass the stop line for the permitted movement, even if the main red light remains illuminated. This is one of the points that catches learners out: a red circular signal normally means stop, but an illuminated green directional arrow authorises the movement it indicates.

That permission is not the same as having a guaranteed clear route. You must still assess the junction, look for vulnerable road users and make sure you can complete the manoeuvre safely.

Why you still need effective observations

Filter arrows control traffic, but they cannot remove every hazard. Pedestrians may be crossing a side road, cyclists may be approaching in a cycle lane, and another road user may be somewhere you do not expect them to be.

This matters particularly when turning left. Before moving, check your left mirror and blind spot where appropriate, then look through the turn. A cyclist may be travelling alongside you, even if you have been waiting at the lights for several seconds.

When turning right, look through the junction towards your intended road. Although a right-turn filter phase often holds opposing traffic, never rely on that alone. A vehicle may have entered late, an emergency vehicle may need space, or the exit may be blocked by stationary traffic.

The safest routine is simple: read the arrow, check the direction you are travelling, make your observations, and move only when you can clear the junction. A green filter arrow is an instruction you can trust, but safe driving still requires you to use your judgement.

Filter lights and lane discipline

At signal-controlled junctions, the road markings are just as important as the traffic lights. Direction arrows painted on the road usually show which movements are allowed from each lane. Get into the correct lane early, rather than making a late change near the stop line.

For instance, a junction may have a right-turn lane with its own right-pointing filter arrow. If you are in a lane marked for ahead traffic, do not turn right simply because you can see the arrow operating for the lane beside you. The signal may not apply to you.

Look for the following before you reach the junction:

• lane direction arrows on the road surface
• signs showing which lane serves each destination
• separate signal heads above individual lanes
• cycle lanes, advanced stop lines and pedestrian crossings

Good planning makes filter lights much easier to handle. During driving lessons, practising busy junctions in areas such as Barnet, Enfield or Watford can help you recognise lane layouts early, rather than trying to process everything when the lights change.

What happens when the filter arrow goes out?

A filter arrow may operate for only part of the signal sequence. When it goes out, the permission to travel in that direction has ended. If the main traffic light is still red, you must not continue over the stop line for that movement.

Do not try to squeeze through because the driver behind seems impatient. If you are already committed to the turn and can safely complete it, continue. If you are waiting behind the stop line when the arrow disappears, stay where you are and wait for the next safe signal phase.

At some junctions, the main light will later change to a full green signal, allowing several movements to proceed. Even then, the rules do not change: follow the lane markings, give way where required and do not enter the junction unless your exit is clear.

Common mistakes learners make at filter lights

The most common mistake is treating a filter arrow as a general green light. It is not. The arrow is direction-specific, so you must only use it for the movement displayed.

Another mistake is waiting unnecessarily because the main light is red. If the green arrow applies to your lane and intended direction, you are allowed to go. Hesitating is understandable at first, but it can create uncertainty for other road users and may hold up traffic.

Some learners also focus so closely on the signal that they forget their mirrors and observations. This is especially risky at left turns, near cycle routes and at multi-lane junctions. The light tells you when you may move; your observations tell you whether it is safe to move at that exact moment.

Finally, avoid entering a junction when the road you are turning into is blocked. A green arrow does not give you permission to stop across a pedestrian crossing, obstruct another road or become stranded in the junction. Wait until there is space to complete the turn.

How filter lights may come up on your driving test

Your examiner will expect you to respond to a filter light confidently, legally and safely. They will be watching how you approach the junction, choose your lane, respond to the signal and carry out observations before moving.

There is no need to make an exaggerated show of looking around. Use natural, timely checks. Slow down smoothly as you approach, read the road markings and traffic lights early, then position the car correctly. If the filter arrow appears, proceed when it is safe. If it does not apply to your direction, remain stopped.

A minor hesitation is not automatically a fault. However, repeatedly missing safe opportunities or failing to act when a signal clearly permits your movement can affect the flow of traffic and may be marked if it becomes unnecessary hesitation. Your goal is controlled progress, not speed.

A quick way to remember filter arrows

When you see a green filter arrow, think: ‘This direction, this lane, when safe.’ That short reminder covers the essential decision-making process. Check that the arrow matches your intended route, confirm it applies to your lane, then use your mirrors and observations before moving.

With practice, filter lights stop feeling like a trick question. They become another useful part of reading the road ahead. Build that habit on every lesson: look early, understand the signal, protect vulnerable road users and make a positive decision when it is safe. That is how confident drivers keep traffic moving while continuing to drive safe and smart.