How these roads are rated & measured

How roads were selected

Candidate roads came from local forums and guides — r/Touge, r/bayarea, r/SanJose, Rennlist, the Bay Area Riders Forum, CivicX, Best Biking Roads and Pebble Beach — plus road-specific and posted-limit searches. Several forum pages blocked direct retrieval; their indexed excerpts were used for discovery only, never as evidence for a current speed or access claim.

Each road links to the discussions it came from, so you can judge the sourcing yourself rather than taking our word for it.

Difficulty ratings

Difficulty is an editorial assessment of road width, bends and sightlines. It is not a community vote, an official assessment, driving instruction, or a safety guarantee.

• 1 — Relaxed. Relatively open or gentle.
• 2 — Winding. Sustained curves at a steady pace.
• 3 — Demanding. Narrow sections, hairpins, blind crests or complex mountain geometry.

Road character is a separate axis and deliberately carries no mph target. Technical means tight or complex; Low, Medium and High speed describe relative road character, still subject to every local limit. Character is what the map colours, because it is what the map filters by; difficulty is shown as a number so the two axes never compete for the same visual channel. Difficulty 3 is called “Demanding” rather than “Technical” so the two scales do not share a word.

How bends are counted

Road geometry comes from OpenStreetMap ways, fetched with the Overpass queries checked into the repository. The build clips traces at existing vertices, joins exactly matching endpoints, and preserves disconnected pieces — it never draws a straight bridge across a gap.

Because OpenStreetMap vertex spacing is uneven, each trace is resampled at a fixed 20 m step before any corner is counted. Then:

• Bend — a run of sustained same-direction turning of 40° or more. Direction wobble under 3° is treated as digitising noise and does not split a sweeping curve into several.
• Switchback — a bend that carries through 130° or more.
• Turning per mile — total absolute direction change divided by length. A scale-free measure of how curvy a road is, used for the rankings.

These are measurements of a mapped centreline. They are not a difficulty score, not a safety assessment, and not an indication of how fast any road should be driven.

Elevation

Each road is sampled every 100 m along its centreline and looked up in USGS 10 m elevation data, a public-domain federal dataset, via OpenTopoData. Samples are smoothed over 300 m before anything is measured, because a digital terrain model carries noise that would otherwise be counted as climb.

Total climb counts vertical change in both directions. Summing only the ascents would depend on which way round the road's geometry happens to be stored — the first version of this measurement reported Page Mill Road as climbing 124 ft, because its trace runs downhill. A road is driven both ways regardless.

A terrain model describes the ground, not the roadway. Where a road tunnels, bridges or runs on a cliff shelf, the model reports the hillside instead: Devil's Slide on Highway 1 initially measured as a 35% grade. Steps implying a grade above 20% are therefore treated as terrain rather than road, and the steepest-grade figure is the 95th percentile of half-kilometre stretches, not the maximum, so a handful of cliff samples cannot set the figure for a whole road.

Elevation profiles are drawn along the longest continuous section of each road, and say so where that is less than the whole. Splicing disconnected pieces together would draw a climb nobody can drive.

Speed evidence

A published source is not a current field-verified sign, and this site never treats one as the other. Every entry scopes its claim and links its source. Specifically:

• Published limits cite a county code, ordinance or agency document, scoped to the section that document actually describes.
• Mapped limits are community OpenStreetMap maxspeed tags, shown with the approximate share of the trace they cover, and explicitly not checked against current signs.
• Unverified means no reliable posted limit was found. We leave it blank rather than guess.

Speed limits are never invented by averaging tags, inferring from road shape, reading a routing duration, or quoting a driver's reported speed. Research turned up a 2012 Panoramic Highway draft ordinance and a 2008 Mines Road proposal; because adoption could not be established, neither is presented as a current posted limit. Recent reporting notes 2026 changes on Highway 9 — another reason mapped tags must not be read as current signage.

Posted signs always govern.

What this site is not

• Not live road status. Check Caltrans QuickMap for closures and conditions.
• Not navigation. Traces show selected road sections, not complete driving itineraries. 17-Mile Drive's named-road trace is not the full signed scenic loop.
• Not a guarantee of access. Park hours, private-road fees and seasonal closures apply on several roads.

Attribution

Road geometry and maxspeed tags are © OpenStreetMap contributors, available under the Open Database License. Full details in the data README. Descriptions, difficulty ratings and corner counts are ours.