What is a phone HUD?
Lay your phone face-up on the dashboard and its screen reflects in the windscreen. Everything here is mirrored so the reflection reads the right way round, in green on black, which is the least distracting combination at night and what every factory HUD settled on.
The speed floats in the glass at roughly the distance you're already looking, so reading it costs no refocusing and no glance away from the road.
How do I set it up?
- Park first. Set everything up before you move.
- Press Start, allow location and wait for a fix.
- Pick your unit and speed alert while you're stopped.
- Lay the phone face-up on the dashboard, screen towards the windscreen, roughly in line with the wheel.
- Turn the brightness up and check the reflection. Slide the phone forward or back until it sits below your natural line of sight.
- Use a non-slip mat. A phone sliding across the dash under braking is a hazard on its own.
Why green on black?
Two reasons, both about your eyes rather than looks.
Bright broad-spectrum light wrecks your dark adaptation. A narrow band at the green end doesn't, and a black background emits almost nothing, so only the digits are lit. Same reasoning behind red instrument lighting in aircraft and on ships.
Your eye is also most sensitive to green in low light, peaking around 555 nm and shifting greener still as it gets darker. So a green readout is legible at lower brightness than white, which means less light in your eyes and less scatter across the glass.
Why do I see a double image?
Car glass is laminated: two layers bonded around a plastic interlayer. Each surface reflects, so you get two images very slightly offset, one a faint ghost of the other. Factory HUDs use a specially wedged windscreen to make them coincide. A phone can't, so expect a slight double. It's worse with large text, which is one reason the readout here is a single line rather than a panel of figures.
Reading it safely
- Position it low. The reflection should sit below your line of sight, over the bonnet, not in the middle of the road ahead.
- Keep it to one number. A HUD works because it costs no attention. Fill it with information and it becomes another screen to read.
- Set the alert before you drive. The point of the beep is that you don't have to look.
- Never adjust anything while moving. Pull over.
Battery and heat
Full brightness plus continuous GPS is about the most demanding thing a phone does, and a dashboard in the sun is a bad place for a battery. Keep it plugged in, and on a hot day put it somewhere the sun isn't landing.
About the lag
A fix arrives about once a second, so the number is always a fraction of a second behind. Cruising, you won't notice. Braking hard, you will, and the displayed speed trails the real one. This is a reference display, not something to brake by.
Frequently asked questions
Why is the text backwards?
Because it's meant to be. The screen faces the windscreen and the reflection reverses it the right way round. If you're reading the phone directly, you're not using it as a HUD.
It doesn't work in daylight.
Correct, and no phone-based HUD does. A reflection in glass can't compete with sunlight behind it. Use it at night or in a tunnel.
Why do I see a faint double image?
Laminated windscreens have two reflective surfaces, so you get two slightly offset reflections. Factory HUDs use a wedged interlayer to merge them. A phone can't.
Is a phone HUD legal?
Rules differ by country and often by state. Many places restrict anything displayed in the driver's field of view. Check your local law, and never handle the phone while driving.
Can I change the unit or set an alert?
Yes. Do it before you set off. The alert beeps, which is the point.