== this info is as of Dec 2008 ==
This was a week long vacation for Christmas and I had planned to drive down to madurai from bangalore. For more than a week, I was working on the route to take, the road conditions etc. Unlike other topics, this requires up-to-date information; no matter how good the info is, it becomes stale or is unreliable (you know how frequent is the changes in the indian roads).
The ideal route from Bangalore to Madurai would be Bangalore -> Hosur -> Krishnagiri -> Salem -> Namakkal -> Karur -> Dindigul -> Madurai. This route is ~430 kms long.
I read that there is heavy road work in progress after Karur, so decided to detour via Trichy (Namakkal -> Musiri -> Trichy -> Melur -> Madurai). This route is ~50 kms longer than the original route. I regretted later for choosing this route.
The road from Bangalore to Hosur starts the trip with a bad note. The road strip is pathetic. Lots of rough patches and the under-construction bridge takes most of the space on the road. But once the bridge opens, this stretch is going to be a breeze (atleast in the early morning). Frequently I had switch between the main road and the service roads due to the construction work. It took around 45 minutes to reach Hosur.
Then comes the wonder (atleast in India). Right after hosur, the four lane express highway starts. This is totally complete until krishnagiri and is 90% complete till Salem. There is very less crowd in the early morning, and the qualitiy of the roads and the view of a lonnngg road gives you the confidence to step on the pedal. There is a toll gate on the way for the highway. They charged Rs.25 for cars (one way). This was very pleasant and I managed to reach Salem in 2.5 hours (this is 200kms from Bangalore, not to forget the 45 mins spent on the first 35 kms). This was a bit of overspeed, but there was no reason why I shouldn't.
Have a look at the highways and see for yourself. Only one way is visible. There are 2 more lanes like this on the other side of the divider.
Would you mind speeding on these roads? ;) again only one way visible.
After Salem there are a number of road diversions where one way roads suddenly become two ways; you will have to join the opposite way and drive against the traffic. It is quite uncomfortable specially when there is no vehicle infront yours. At times, people driving on opposite side do not realize that this road is "now" 2 way. It's quite risky. I would advise to drive with head lights on when you drive on these roads and dip/flash your headlights as required. Salem->Namakkal strip is quite complete except for a few diversions. The diversion boards are missing at some points and people drive at their own choice of roads. If you are not used to the route, avoid night driving.
Come back to reality after Namakkal. I had to go through the namakkal city to reach the trichy highway. Namakkal to Musiri highway was quite ok. Less traffic, good roads (but only one lane per way). Musiri to trichy was a mess. Bad roads and were very narrow. I had to be careful about cows and sheeps with no herders crossing the roads every once in a while. No chance to speed. Namkkal to trichy took a solid 1.5 hours for ~80kms. The highway enters trichy at toll-gate. The by-pass from toll-gate to trichy railway junction is under construction. This is not going to get ready for atleast a year. I got diverted very early from the by pass into the city. sucks! The roads were pathetic all over the route from toll-gate to madurai highway (tollgate --> thiruvanai kovil -> sathiram bus stand -> thillai nagar -> trichy junction -> madurai highway). With the bad roads and the peek city traffic at noon, this took a solid 1 hour. Waste of mileage and time. Please do not consider this detour. Trichy to madurai stretch was neither that pleasing. 4 lane highway is under construction in most places and is around 40% complete I would say. Roads are acceptable except after Melur. Melur to madurai stretch was pathetic with lot of potholes on road.
On the way back, I took the normal route (Madurai -> Dindigul -> Karur -> Namakkal). To be frank, overall this route is much better than the detour. Madurai->Dindigul route is in bad shape. Bad roads. The 4 lane highway is barely started. But after Dindigul, things start getting better and the highway gets better and better as it reaches Namakkal. Madurai -> Dindigul took around 1,45 hours while I reached Salem in a decent 5 hours. On the way back, Salem->Hosur highway was little crowded with the trucks playing the spoil sport. They occupy both the lanes at 50kmph and are the real pain in the a$$. I could still reach Hosur in ~3 hours. After all this, bangalore welcomes you with hell a lot of traffic right from Electronic city (just after Hosur). Its horrible as of now. I wish the bridge gets ready soon.
I hope this info would help someone who would travel in this route in the next 1 or 2 months. Once the 4 lane highway is totally complete till madurai, it is going to be a matter of ~6 hours drive.