For those seeking a route-planning tool, here is a fairly comprehensive description video by the developer.

I have used the site for years to design a route then transfer the gpx files to my Garmin.

What a great resource!

I was just thinking the other day that it would be great to put up a route planning / route advice web resource that allows one to reach local cyclists to ask about options, get opinions, etc.  Any fully-automated tool is going to be limited by the available data (I'm sure most of us have met with disappointment when our favorite tool has headed us onto what it claims is a paved surface, only to learn that it's anything but) so there's no substitute for actual eyes-on local knowledge.

Thanks! Yeah I'm always interested in these. While it's hard to beat a big paper map for route planning (or local knowledge) I certainly don't always have these. 

In the UK, I use Ordnance Survey maps for planning, especially if I'm considering any off-road use. These are extremely detailed (at high resolution they show every pylon and hedge boundary) - the downside being it used to be extremely expensive to buy them for an extensive area. But now Bing maps makes available coverage of the whole country for free! Just activate the Ordnance Survey layer and zoom in.

For discovering over people's routes, getting (usually pretty reliable) suggestions, and seeing surfaces I've had a lot of success with Komoot. You can now do all this via the browser so there's no need to use their app.

Then for sketching routes out, the RideWithGPS route planner is pretty good. It occasionally struggles with some of the sketchier bridleways/tracks - one trick is to change to "walking" mode which usually allows them to be followed.

I've been using RideWithGPS for a year and a half.  When planning routes I turn on the Global Heat Map option, so I can see the fine red spider web of roads that others have ridden regularly.  The heavier the line the more the road's been ridden.  

I typically use the OSM Cycle base map layer, but sometimes switch to one of the others when OSM Cycle doesn't have the topological data needed to connect two points via an obscure sidewalk or some other cut through.  When I'm forced into doing so, I'll switch to their "connect the dots" line drawing mode for places where I know darn good and well that I can get from Point A to Point B but the website can't get it to happen.

I'll have a look at Komoot, too.  Thanks for the tip.

The U.S. equivalent to the venerable Ordnance Survey maps are the US Geologic Survey topographic maps.  The are available at several scales; I find the 1:100,000  series good for "big picture" planning, then either the 1:50,000 or 1:24,000 versions for the really fine details.  Like the Ordnance Survey maps, the 1:24,000 (7' 30" on each side) maps show power line pylons, gas lines, railroad tracks, and pretty much everything else.

It'd be prohibitively costly to buy the entire series of either one in paper format, and equally prohibitive to try to carry all of the 1:24,000 quadrangles you'd need for a long tour, but they're available for download at no cost.  Living where I do, I can also belt down to the Library of Congress Geography and Map Division and have them fetch as many of the paper ones as I need from their collection for free, gratis, and at no cost to me.

They're also available as one of the base map options in RWGPS.