I'd be very wary of using Sustrans routes. Some take you ridiculously out of your way and some go through areas that I wouldn't cycle through after midday let alone in the evening. Your best bet is to use Google Maps combined with Bing maps zoomed to OS level and combine it with a healthy dollop of local knowledge, either your own or from a friend/colleague.
Good luck.
Thats a very good point Caph, I remember two years ago, turning back on the cycle path to bristol as I suddenly didnt feel safe, and that was in the morning.
As a usually solo cyclist, and a woman, I always listen to my instincts, like I have learnt to when wildcamping alone in my m/home....
On that occasion i turned around and headed into Bath and felt totally safe and thoroughly enjoyed it.
Your post serves as a timely reminder that we must always be aware of possible dangers and try to stay one step ahead of potential dangers.
Happy cycling
Lynda