Using HUD mode safely

A heads-up display puts your speed in your field of view so you are not looking down at a dashboard. Cars have shipped them since the late 1980s; you can approximate one with a phone running the live speedometer, a flat dashboard and a dark night.

How the reflection works

Lay the phone face-up on the dashboard. Its light hits the inside of the windscreen and part of it reflects back toward you, appearing to float somewhere out over the bonnet. Because you are looking at a mirror image, the digits have to be flipped horizontally on the screen so they read correctly in the reflection. That is what the Mirror button in HUD mode does, and the only reason the phone itself then looks backwards.

Getting a usable image

• Darkness is essential. Windscreen glass reflects only a few percent of the light hitting it. In daylight, sunlight through the same glass overwhelms that completely. HUD mode is a night feature.
• Maximum brightness, with auto-brightness disabled so it does not fade.
• Clean the inside of the windscreen. Dust and film scatter the reflection into a smear.
• Flat, non-slip surface. A silicone mat stops the phone sliding under braking.
• Expect a double image. Laminated windscreens have two glass layers and reflect twice. Most cars show a faint ghost a few centimetres from the main image; short of a purpose-made combiner film there is nothing to be done about it.
• Angle matters. Move the phone closer to or further from the base of the windscreen until the reflection sits where you naturally look.

Safety, plainly

Set everything up before you drive. Choose your units, tap Mirror, start tracking, and put the phone down. If something needs changing en route, either let a passenger do it or pull over.

Two more things worth saying:

• A phone loose on the dashboard becomes a projectile under hard braking or in a collision. Secure it, or accept that risk knowingly.
• In some jurisdictions any screen visible to the driver is regulated regardless of what it displays. Check your local rules before relying on this.

Why this is free here

Every other online speedometer that offers HUD mode puts it behind an app store download. It is a horizontal flip and a large font. It has no business being a paid feature, so here it is a link in the navigation.

Keeping the screen on

HUD mode requests a screen wake lock, so your display will not sleep mid-journey. That does mean the screen is on and bright for the entire drive, which is hard on the battery — plug in for anything longer than a short trip.

Sources

• W3C Screen Wake Lock API is the mechanism HUD mode uses to keep the display from sleeping.
• WHATWG Fullscreen API is what puts the mirrored digits full-screen with no browser chrome.
• GOV.UK: Using a phone, sat nav or other device when driving sets out the UK rule: hands-free use in a dashboard holder is allowed, handling the device while driving is not.