<p>osm.org specifies a constant zoom level of 14 because OSRM is more likely to find a route that way: <a href="https://github.com/Project-OSRM/osrm-backend/blob/develop/data_structures/static_rtree.hpp#L580">https://github.com/Project-OSRM/osrm-backend/blob/develop/data_structures/static_rtree.hpp#L580</a></p>

<p>At z15 and above, OSRM will potentially snap onto a "tiny component", i.e. a small, unconnected piece of routing graph from which you can't route anywhere else. At z14 and below, tiny components are ignored.</p>

<p>You could take the view that tiny components are actually useful for debugging routing errors in OSM, of course, but weighed against that is the consistently worse user experience of routing when a significant proportion of queries fail.</p>

<p>I thought that there was a plan to break out the "tiny components" detection into a separate parameter, but looking at the OSRM source it still appears to be hardcoded to z<=14.</p>

<p style="font-size:small;-webkit-text-size-adjust:none;color:#666;">—<br>Reply to this email directly or <a href="https://github.com/openstreetmap/openstreetmap-website/issues/1077#issuecomment-151191895">view it on GitHub</a>.<img alt="" height="1" src="https://github.com/notifications/beacon/ABWnLdkLfaOjgqTeF0gc-UvDiPJEMc-Eks5o_khkgaJpZM4GVDtH.gif" width="1" /></p>
<div itemscope itemtype="http://schema.org/EmailMessage">
<div itemprop="action" itemscope itemtype="http://schema.org/ViewAction">
  <link itemprop="url" href="https://github.com/openstreetmap/openstreetmap-website/issues/1077#issuecomment-151191895"></link>
  <meta itemprop="name" content="View Issue"></meta>
</div>
<meta itemprop="description" content="View this Issue on GitHub"></meta>
</div>