arindam_xeta , it seems that we both took the Dumka - Bhagalpur alternate route within a span of 15 days, driving Hatchbacks , and had entirely different experiences. It seems to boil down to timing and when you leave Kolkata or Siliguri. You will need to plan and cross the cities / towns along the route in the non-peak hours to gain maximum benefit.
I had used my GPS extensively , but used down loaded maps for the whole journey to compensate for occassional loss in signal at crucial moments. Here is the kml file of the entire route in case anyone needs it , I had it loaded before I started my journey.
KolkatatoSiliguriviaBolpurSiuriDumkaBhagalpur.kml
After reading your post , I was going through my notes again and here are my observations :
From Airport 1 No gate till Purandarpur , which is about 180 Kms , the road is good all the way , no cause for complaints except for 1-2 kms on the bridge near Kopai Resort (just after Bolpur), which is in disrepair. You will also need to take the detour near Bhedia to escape the damaged railway underpass in the main GT Road.
12 Kms from Purandarpur till you cross Siuri, it is sometimes narrow and passes through heavy town traffic , but the road is good.
7 Kms - as you leave Siuri till Seorakhuli More is moderately bad., like any other "normal" SH
8 Kms - from Sheorakhuli More till Jharkhand border was pothole ridden , but is patch repaired , hence moderate speeds
7 Kms - about 7 kms after Massajore dam (going towards Siliguri) and upto the T junction near Patabari is bad with Potholes
11 Kms - from Patabari till Kuruwa / Sristi Park Gate is Nothingness - no surface and full of craters, the worst section on this route.I took the left turn at the Kuruwa Gate to completely by pass Dumka Town.
3 Kms - from this turn to Dumka Airport gate, it is very narrow and in shambles , then the road gets excellent upto the Clock tower at Dumka
48 Kms - from Dumka clock tower till the Bihar border at Valjore is moderate with frequent potholes and in-frequent craters (about 20Km of this 48Km is really in bad shape). Next 63 kms from Valjore to Bhagalpur is great tarmac.
24 Kms - as you enter Bhagalpur till you meet NH31, is very slow city roads in bad condtion and a bumpy ride over Vikramshila Setu , not great , just very slow.
Next 75kms Kms on NH 31 through Purnea and upto the point where it becomes 4 laned is also in excellent condition.
There is no 4 laning for about 32 kms between Kishanganj and Islampur , road is in good condition , but traffic through these congested towns is slow moving during peak hours.
5 kms - Near Bagdogra it is again in it's usual devastated state with huge gaping craters.
On an average it took me 14 and a half hours on both ocassions with breaks totalling about 120 - 180 mins for each leg. I had made an early start on both days , 4 AM from Kolkata and 3 AM from Siliguri.
Personally I will prefer it over NH34 (in it's present condition) any day , and there are thankfully no traffic bottlenecks in this route save the few congested towns. It is longer by about 100 Kms compared to the normal route , but only about 50 Kms of the total 600 + odd kms will really tax you.