Bad route: Sending me down 24% grade hill

[graue]

Routing from Gallardo's to the Connecticut Friendship Community Garden, both the bike-only route and the bike-on-Muni 19 route I'm getting send me down a hill that's a 24% grade for the first block (20th St east of Rhode Island) and then another 20% or so grade for the block after that.

I consider this unsafe to go down and will walk my bike down which is tiring and slow:

Reasonable route is 17th St (bike lane) east to Connecticut, then Connecticut south, which never goes above ~13% grade on the steepest block according to HillMapper.

Or, after crossing the 18th St ped bridge, backtrack north on Vermont to Mariposa, east to Connecticut. That's hillier but avoids 17th St, the main road with unprotected bike lane

I'm curious why BikeHopper is choosing such a bad route. Do we have inaccurate elevation data here? (The elevation measurement applied on the frontend says "max grade 11.8%" for the 20th St segment, so it's off by a factor of two.) Do we not disfavor steep downhills enough?

[rsarathy]

The segments in question on 20th St have grade: 12 when viewed through the debugger.
Downhill grades in the interval [0.12, 0.15] have their base penalty ticked down three levels.
The corresponding way for 20th St has the tag highway=residential, giving a base penalty of SLIGHT_PREFER (2.5).
This means that the 20th St segments' resultant penalties are considerably favorable; I see penalty: 2 in the debugger.

I think we're bumping into limitations of our elevation data again. We could also adjust the grade boundaries which determine penalties, such that we decide that a steep downhill of maybe 10%+ is not preferable. Currently, only downhill grades ≥ 16% result in an increased penalty (REACH_DESTINATION / 12.0).

[graue]

Thanks for finding that info! I think in addition to this highlighting the need for better elevation data, downhill grades over 10% should be discouraged somewhat.