Ralph, I'm not aware of any way to specifically ID canals other than to actually look at the map. Changing the map style may help. 

You could try plotting a route using the 'night time' feature. As I understand it, it should avoid canals - but would also avoid other places that might be pleasant in the daytime. 

I'm guessing a lot of canals are not paved so  selecting 'Paved' might help. 

It would have been useful if you'd shared an example.

Thanks. I do sometimes swap to OS map when planning, which is good for identifying canals. But sometimes I do planning on the go, with poor Internet coverage, so OS takes a while to appear (if at all) and I abandon that check. The most scary recently was going from Chirk (near Wrexham) to Llangollen (North Wales) on the canal, specifically the Whitehouse Tunnel.  I thought it was just a wide & low bridge when I went in (pushing my bike) but then realised it wasn't - but it would have been difficult to reverse due to the narrow path leading into it so I carried on, getting darker and darker, then met a group of other cyclists pushing their bikes towards me half way through. We had to do careful contortions to get past, lit by someone's phone torch. I noticed afterwards that it does say 'tunnel' on the normal mapping if you zoom right in, but I hadn't thought to do that beforehand. Still, it made for a memorable journey:-)

Ralph, I'm sorry but I'm going to be 'that guy'. 

Planning on the go, assuming the website, you´ll see this:

Not only is the tunnel highlighted, the advice is to push!

(If you use the app, you'll get the same info by opening the route on the website on your phone)

If you'd looked at StreetView you'd have seen this:

If you went in there with a loaded bike without doing a walk-through in advance, then that's on you. 

Finally....

We had to do careful contortions to get past, lit by someone's phone torch.

You set off for a day without a working light? On a laden bike? Seriously?

I'm all for exploiting the freedom that a bike gives us and the less planning the better as far as I'm concerned. However, there is a huge difference between planning and preparation.  If you want to plan on the go, then my very strong recommendation is to be prepared for what can reasonably be expected. Darkness can be expected. A headtorch is your friend. 

Osmand is a useful app for offline maps and planning to supplement CT. A steep learning curve but very useful for the 'less planning, more preparation' attitude. 

Thanks for the suggestions. I use CT alot (about 9,000 miles over the past 3 years) and haven't used the turn-by-turn feature so far, and the tunnel was the one and only time I've felt any real concern, so that's a real testimony to CT, I think. I generally just put in To and From, then add via points for coffee, supplies etc and to avoid steep climbs where poss, then compare with and without 'Paved only'. I think checking OS for canals is probably the way I will go, now.

Re: lights - I have a SON hub dynamo and LED front lamp but, whilst brilliant(!) at cycling speed, it blacks out below about 2mph, and I was pushing very slowly out of necessity, so no light. My head torch was in my panniers. So yes, it was my fault really. Anyway, I still think CT is wonderful, and recommend  it to anyone who will listen.

I rode that exact tunnel this weekend! As I remember, it is quite heavily signposted (along with alternate directions for people who wish to avoid it), so this is probably a reminder to look at the world around you and not just your map :p

Just to add I would suggest exactly the same as Hobbes – the ‘Night’ mode (in the UK) will avoid towpaths.

Whitehouse Tunnel I’m not so worried about but Pontcysyllte aqueduct is something else entirely!