In a tunnel, a GPS app can’t measure your speed at all. Satellite signals don’t pass through rock and concrete, so the app either freezes on the last number, drops to zero, or estimates. Between tall buildings the signal is there but bounces off walls, which makes speed jumpy and distance zigzag. In both places your car’s or motorcycle’s own speedometer is the number to trust, because it reads the wheels, not the sky. Neither problem is a fault in your phone, and a well-behaved app tells you plainly when it has lost the signal.
Why does GPS stop working in a tunnel? #
GPS satellites broadcast from about 20,200 km up, and by the time the signal reaches you it’s extremely weak. It gets through a car roof, a jacket pocket or a thin layer of leaves, but not through metres of rock, earth and reinforced concrete. A tunnel causes three problems in turn:
- At the entrance, satellites drop out one by one behind the portal, and the last few fixes get less accurate.
- Inside, there are no fixes at all. The phone doesn’t know where you are or how fast you’re going.
- At the exit, the receiver needs a few seconds to pick satellites up again, and the first readings can jump before they settle.
Multi-storey car parks, covered bridges, stacked interchanges and deep cuttings cause the same effect on a smaller scale.
What does a speedometer app show in a tunnel? #
| What the app does | What you see | The risk |
|---|---|---|
| Holds the last value | Your entry speed, unchanged | You think you’re doing 80 when traffic has slowed you to 50, or the reverse |
| Drops to zero and says so | 0 plus a “searching” or “no signal” message | None, as long as you know to ignore it |
| Estimates from the map or motion sensors | A number that drifts from reality the longer the tunnel | Looks trustworthy but isn’t |
Navigation apps usually keep your arrow moving, assuming you continue along the road at roughly your last speed, sometimes helped by motion sensors. That’s fine for knowing where the exit is, and it’s why the arrow sometimes pops out ahead of or behind your real position. It isn’t a measurement of speed.
Speedometer GPS takes the second approach deliberately. If no valid fix arrives for five seconds, the readout drops to 0, the dial dims and the accuracy indicator shows it’s searching. Near a tunnel mouth, where fixes may still arrive but are coarse (worse than ±20 m), the indicator turns red and those fixes aren’t used for speed or distance. When the phone sees the sky again, readings return within a few seconds, starting from fresh data rather than an average polluted by the gap.
Can phone motion sensors keep measuring speed? #
Only for seconds. You can estimate speed by adding up acceleration from your last known speed, a technique called dead reckoning, but small sensor errors pile up fast, and a phone bouncing in a car can’t tell braking from a pothole. After a minute the estimate can be several mph off. Car navigation systems that track well through tunnels read the car’s wheel speed, which a phone app can’t.
How should you drive or ride through a tunnel? #
- Check your speed on the vehicle’s own speedometer before the portal. That’s the number that stays valid inside, and since it reads slightly high by design, it errs on the safe side.
- Read the signs on the approach. Many tunnels have their own, lower limit and speed cameras.
- Don’t look at the phone inside. There’s nothing useful on it, your eyes are adjusting to the dark, and a speed alarm can’t warn you with no speed to compare. Treat the tunnel as alarm-free.
- On a motorcycle, flip a tinted visor up or slow down before you go in, keep your following distance up, and watch for dust or damp near the portals.
- After the exit, give the GPS a few seconds to settle before you trust it again.
How much trip distance do you lose in tunnels? #
Every metre of tunnel is a metre the GPS can’t see, and how an app handles the gap decides how much goes missing:
- Apps that join the dots connect the last fix before the tunnel to the first one after it. For a straight tunnel that’s close; for a curved one, a little short.
- Apps that reject big jumps throw the tunnel out, because a sudden jump of hundreds of metres looks exactly like a GPS glitch on a normal road.
Speedometer GPS does both, depending on length. When the signal returns, it adds the straight line from your last good position to the new one if that gap is under 200 m, so short underpasses cost almost nothing. Any jump longer than 200 m is discarded, so a 1.5 km tunnel goes uncounted and your trip reads about 1.5 km short. Trip time keeps running through the tunnel, because it’s clock time since you pressed Start; moving time pauses while there’s no fix.
For a commute with 2 km of tunnels in a 25 km drive, that’s about 8% short. On an Alpine route or a coastal road with several long bores, expect a gap of roughly the combined tunnel length; the vehicle’s odometer, counting wheel turns, won’t have it. If you need the full number for a mileage log, note the tunnel lengths from the entrance signs and add them, or use the odometer for that route. Logging commute distance has more on city trips.
Why is GPS speed jumpy in cities? #
Streets lined with tall buildings are called urban canyons, and they cause two separate problems.
Fewer satellites. Buildings block most of the sky except a strip overhead. With the visible satellites bunched together, each position is less certain, and the accuracy radius grows from a few metres to 15, 30 or more.
Reflected signals (multipath). Glass and steel reflect satellite signals. A reflected signal travels further than the direct one, so the phone miscalculates its distance to that satellite and puts you in the wrong place, sometimes across the street. As reflections come and go, your position jumps, and each jump can look like a burst of speed.
Phones measure speed mostly from the Doppler shift of the signals, not by comparing positions, which is why GPS speed is usually steadier than GPS position. Reflections still corrupt it: you’ll see a speed that jumps 10 or 15 km/h for a second while you roll steadily through traffic, or a recorded track that zigzags across the block. Many recent phones receive a second GPS frequency, often called L5, that’s better at rejecting reflections. It narrows the gap in cities without closing it.
How Speedometer GPS filters bad fixes #
- The accuracy indicator shows the live radius: excellent within 5 m, good within 10 m, fair within 20 m, poor beyond that, and searching when there’s no fix.
- Poor fixes aren’t used. Anything worse than ±20 m is shown but ignored for speed and distance.
- One bad reading gets diluted. The display averages the last five one-second readings.
- No stale numbers. Five seconds without a fix and the speed goes to 0 instead of freezing.
- No drift at red lights. Moves under 5 m between fixes are ignored, or under 10 m once you’ve been sitting still.
- No teleporting and no phantom top speed. Jumps over 200 m are thrown out, and top speed comes from the smoothed value.
The honest limit: the accuracy radius is the phone’s own estimate, and a reflected signal can produce a fix that claims ±8 m and is further out than that. The filters remove most of the damage, not all of it.
What to expect in different places #
| Where you are | Typical fix | What happens to speed and distance |
|---|---|---|
| Open highway | Excellent | Speed typically within a couple of km/h |
| Suburban streets, some trees | Good to excellent | Small wobbles; distance close to the map |
| Downtown between high-rises | Fair to poor | Speed jumps; distance can read long from a zigzag track |
| Under a bridge or overpass | Brief dip | Usually back before the five-second cutoff |
| Long tunnel | Searching | Reads 0; tunnel length missing from trip distance |
| Underground garage | Searching | No reading until you’re outside; start the trip there |
How to get cleaner readings in the city #
Do all of this before you set off, not at a red light.
- Mount the phone high, near the windshield, where it sees the most sky. A cupholder or the passenger seat loses satellites the moment buildings close in. On a bike, the handlebars beat a pocket.
- Turn on Precise Location for the app (iPhone: Settings > Privacy & Security > Location Services). Without it, apps only get an approximate position, which is useless for speed.
- Get a fix before you head into the canyon. A phone that already has a lock copes better once the buildings close in.
- Leave Wi-Fi switched on. You don’t need to join a network; it helps the phone work out roughly where it is and lock onto satellites faster.
- Check for a coated windshield. Some heat-reflecting windshields have a metal layer that weakens GPS. If the fix never gets past fair on an open road, try the area behind the mirror or a different mount.
- Don’t check your speedometer downtown. Do that on an open highway, and read trip averages rather than instant spikes.
If the reading sits at 0 even in the open, see GPS speedometer stuck at 0. For why the dashboard and GPS disagree on an open road, see GPS vs car speedometer.
Frequently asked questions #
Why does my speedometer app go to 0 in a tunnel? #
The phone can’t receive any satellite signal inside, so there’s no speed to measure. Apps that show 0 or “searching” are telling you the truth; apps that keep showing a number are either frozen on the last reading or estimating.
How does Google Maps know where I am in a tunnel? #
It estimates, assuming you continue along the road at about your last speed, sometimes helped by motion sensors. That’s why the arrow can reappear in the wrong spot at the exit, then snap back once GPS returns.
Why is my trip distance short after driving through a tunnel? #
With no signal, the app can’t record the path, and Speedometer GPS ignores any jump of more than 200 m between fixes, so a longer tunnel isn’t added when the signal returns. The trip reads short by about the tunnel’s length.
Is my car’s or bike’s speedometer more reliable than GPS in a tunnel? #
Yes. It reads wheel or gearbox speed, which has nothing to do with satellites. It may read a few percent high, as most do, but it’s the only live reading you have underground.
Why does GPS show me moving when I’m stopped downtown? #
Reflected signals make your calculated position wander by several metres even while you’re parked. Speedometer GPS ignores movement under 5 m between fixes, and under 10 m once you’ve been still, so the drift doesn’t add distance to your trip.