How Glare Map works

What the colours mean

• Red: the sun is within 25° of straight ahead for drivers going the way the arrows point. Glare research puts the start of disabling glare at 19 to 25° from your line of sight, so this is the sun in your eyes.
• Orange: the sun is within 45° of straight ahead and less than 25° above the horizon: near your eyes. A nationwide study of crashes in Japan counted sun glare out to 45°.
• No colour: the sun is behind you, off to the side, below the horizon, or high enough that the roof and the sun visor block it.

Glare only happens one way along a road at a time: a road running west-southwest gets it on winter afternoons for drivers heading west, while drivers heading east have the sun behind them. That's what the arrows show.

Where the sun is

The sun's height and direction come from NOAA's solar calculator equations, worked out for each stretch of road at the date and time you pick, in India Standard Time. They match NASA JPL's Horizons system to within 0.1° across the year, and they include the way the air lifts the sun near the horizon.

One state at a time

India has more than 140,000 km of national highways, too many for most phones to draw at once. So the map opens on one state in full detail, with a lighter outline of the rest of the network around it. Pick another state, or all of India, at the top left; zoom in and the states in view fill in their detail too.

A highway's year

Tap or click a highway to see, for each direction, a chart of the whole year: the months down the side, the hours from 4 AM to 8 PM across, shaded where the sun is in or near your eyes on the 15th of each month. The line above it puts that in words, and the dot marks the date and time on the map.

Trips and when to leave

Pick two towns and Glare Map finds the quickest way along the national highways, at 50 km/h on highways and 80 km/h on expressways. Then it drives the trip for every departure time through the day, every 15 minutes, adding up the minutes with the sun in your eyes as it moves across the sky. The strip shows the result, so you can see when to leave to avoid it. It also tells passengers which side the sun is on for most of the drive, so they can sit on the shady side.

What it can't know

• It assumes an open sky to the horizon. Hills, trees, buildings and lorries can hide a low sun, and clouds, fog and haze can soften it, so real glare is often less than the map shows, especially in the hills.
• The highway data comes from OpenStreetMap. New roads and bypasses may be missing, and a highway's number can be missing in places; where its data breaks for a short gap, often in a town, trips are joined across it in a straight line.
• Trips keep to national highways and expressways, at average speeds, so real routes and times will differ.

When the sun is in your eyes

• Keep the windscreen clean inside and out: dirt and smears scatter the light.
• Use the sun visor, and wear sunglasses; polarised ones cut the glare off the road.
• Slow down and leave more room: the drivers ahead may be blinded too, and so may people crossing.
• With the sun behind you, drivers coming the other way can't see you well. Switch on your headlights.
• If you can, leave a little earlier or later: the trip planner shows how much that helps.

The data and credits

Highways, towns and the states they're split by © OpenStreetMap contributors, from Geofabrik's extract for India, under the Open Database License (ODbL). The highway files this site uses, /india.json, one file per state such as /states/assam.json, and /roads.json, are databases made from OpenStreetMap, and they're available under the ODbL too. India's outline is DataMeet's india-composite (CC0), and the land around it comes from Natural Earth (public domain). Every source and library is listed in the credits.

Your privacy

The towns you pick and the trips you plan stay in your browser: nothing about them is sent anywhere. The site counts visits anonymously. See the privacy policy.