There appears to be only motorway/autobahn between Svilengrad in Bulgaria and Edirna on the Turkish side. Can bikes use this road or must they make a detour into Greece and back into Turkey or do cyclists use the railway to overcome the problem if it exists. There is no ordinary road on my map.
In 2000 I biked from the outskirts of Istanbul into Bulgaria via Edirna. The Turkish tourist office told me the road was very busy and dangerous and I should take a bus. So I hunted down where to get a bus and the next morning, loaded my bike onto a small bus for the terribly short ride to the border. Only to find out that there was NO traffic on the road, and thus not dangerous. I was so pissed and embarrassed (I know everyone on the bus was wondering why I was taking the bus for such a short journey). I bet the guy in the tourist office had never even gone to the border! I got off at the border, did the crossing thing and then biked on from there. No danger that I recall. I do not know if the road was E80- I don't seem to recall it was a freeway. At anyrate, you can certainly get from Bulgaria into Turkey on that route with no problem. :) And do NOT bike into Istanbul - extremely dangerous! If you want more info, email me directly.

My bf crossed the border on foot several years ago, early in the morning. He thought Svilengrad was only a km or so away (it's 11 km) so he decided to walk. After he saw a sign with the distance he decided to hitch. It was, admittedly, only 6 am, but he was only passed by about four or five cars altogether.
I've taken that road more than once. It's not busy but truck drivers have often finished a major trek through Europe. They're tired and this is their last border for a long way, so they just want to get it out of the way quickly. So they're not paying the most attention!
I lived in Istanbul for two years - never biking, admittedly. If you must bike into the city, aim to approach it from the north-east. From the countryside to the area where the Bosphorus joins the Black Sea is less busy than the south-west of the city. Once you reach the Bosphorus, there is a pedestrian sidewalk almost all the way down to the city. It's obviously not legal to ride on the sidewalk but Istanbul pedestrians are not likely to complain unless you are riding quickly at a busy time (ie weekend or late afternoon).

Yeah, we were confused by this, but it is not a problem. If you go into Svilengrad, you can get on the old road to Kapitan Andreevo that runs along the North side of the river. If I remember rightly it joins up with the motorway shortly before the border. After the border, I think we rode about 10km along the hard shoulder of the motorway, but it wasn't a problem.
Travelin Hobo is right, the last stretch of the road into Istanbul (D100), from Silivri on, is very dangerous and unpleasant on a bike.

Dear Travelinhobo, Maenad and Creamcrackered
Many thanks for your excellent advice, it is already down in my little blue book.
I leave March 1st.
Neil.
Take the motorway, noone stops you and if I remember correctly there's quite a wide hard shoulder. I went through in 2005.
Dan
London To Cape Town

There is another border crossing from Bulgaria to Turkey, it only opened about eight months ago so it doesn't appear on any of the maps yet. We cycled that way to Edirne and the road was almost completely empty as very few people know about it. The turn off towards the border is near the Bulgarian town of Topolovgrad, a local will be able to give you directions. It's about 80-90km from Topolovgrad to Edirne as far as I can remember. We cycled into Istanbul and it was pretty scary. We stayed north on the road that runs near to the Black Sea coast, it is very pretty, a quiet road, but very hilly. We dropped down into Istanbul too soon, we should have stayed on that road right to the end as many people told us it was a quiet route into the city. Instead we ended up on the main road on the south coast for the last 50 or so km and it was terrifying!