Starting…
Traffic penalty: how much busy roads get marked down. 0% ranks on curves alone.
Straight or busyTwisty and quiet
Paved
Dirt / gravel
Each colored line is one route (at least the minimum length), colored by its route score. Numbers match the list below; blue-grey lines are everything else.
Top routes in view
Roads load as you move the map.
How scores are calculated
Score
Score = Twistiness × (1 − penalty × busyness). Busyness is 0 for any road at least moderately quiet (under ~5,000 vehicles a day) and rises to 1 for the busiest, so quiet roads keep their full score and the traffic penalty only marks down genuinely busy ones.
Calibration
The settings are tuned against famous US roads and against control areas that should score low. Top route scores with the default settings:
| Road to Hana (HI 360) | 99 |
| Tail of the Dragon (US 129), one 12.8 mi route | 98 |
| Back of the Dragon (VA 16) | 98 |
| Angeles Crest Highway (CA 2) | 94 |
| Big Sur (CA 1) | 93 |
| Moonshiner 28 (NC 28) | 91 |
| Million Dollar Highway, Cherohala Skyway | 89 |
| Beartooth, the Snake (US 421), Twisted Sisters | 85–87 |
| Blue Ridge Parkway, Pig Trail | 79–81 |
| Iowa grid, Lancaster farm roads, Florida suburbs, city arterials | 0–50 |
Routes
Highlighted stretches are strung together through junctions into routes, starting from the best stretch. At each junction a route takes the best continuation: good road first, then the same road, then straight ahead. It stays on a road through any bend (hairpins included) but only switches to a different road when going roughly straight on (within ~70°), so routes follow roads instead of zig-zagging block to block. Up to 1.2 mi of weaker road can bridge two good stretches.
Route score = route twistiness × traffic factor × length factor. Route twistiness is measured over the whole route at once, so short straights between curve clusters don't pull a road that's twisty end to end off the top. The length factor is ×0.5 at 1 mi, ×0.67 at 2 mi, ×0.83 at 4 mi and ×1 from 8 mi. Click a route to download it as a GPX file.
Twistiness
Each road is rebuilt from its map pieces, resampled every 20 m, and its turn radius measured over a ~60 m window. Length spent in turns counts by how tight they are, (130 m ÷ radius)0.8 − 0.25:
| Hairpins (radius under 50 m) | ×1.95 |
| 75 m | ×1.3 |
| 100 m | ×1.0 |
| 175 m | ×0.55 |
| 250 m sweepers | ×0.35 |
| 400 m | ×0.15 |
| Wider than ~650 m | ×0 |
Weighted curvy length ÷ total length over the ~400 m centred on each stretch is mapped to 0–100 with 1 − e−x/0.18. Stretches turning more than 2,500°/km (cul-de-sac bulbs, parking loops) are marked down; real mountain roads top out around 1,500.
Traffic (an estimate)
There is no free worldwide traffic-count feed, so vehicles per day are estimated from the map: a typical rural count for the road type, times a few multipliers. Click any road to see its own breakdown.
| Highway / trunk road | 6,000/day |
| Primary road | 3,500/day |
| Secondary road | 1,800/day |
| Minor road | 900/day |
| Back road | 200/day |
| Track | 20/day |
| Interstate / national route number | ×2.5 |
| US highway / major route number | ×1.25 |
| Other route number | ×1.1 |
| Expressway | ×2.5 |
| Unpaved | ×0.5 |
| Built-up area | ×(1 + 0.6 per point of development above 2) |
| Remote area (development under 1.5) | ×0.75 |
Quietness = 1 ÷ (1 + (vehicles ÷ 5,000)0.85): 300/day is 92, 1,500 is 74, 4,000 is 55, 15,000 is 28.
Development is the km of road (driveways and parking lots included) per km² within ~750 m, or 0.7 × the percent of ground covered by buildings, whichever is higher. Open country is 1–3; suburbs, townhome communities and towns are 6–25. Back roads above 5.5 count as neighborhood streets and are left out of routes unless you include them.
Data
Roads, names and surfaces are OpenStreetMap data from OpenFreeMap's vector tiles (refreshed weekly), loaded at zoom-13 detail, which keeps about 96% of the curve detail of the raw map. Surface is a guess when a road has no surface tag: tracks count as dirt, everything else as paved. Check the satellite view before committing to a dirt route.