The Pamir highway leads from the city of Osh, Kyrgyzstan, through Tajikistan (Gorno-Badakhshan Autonomous Province, Khorug, Dushanbe), via the eastern part of Uzbekistan, to Mazar-e Sharif in northern Afghanistan.
Most of the Pamir Highway is paved, except for the mountain passes. Secondary roads are mostly unpaved and are in very bad condition. The Pamir highway is mostly one-lane. The condition of the asphalt depends; sometimes it is really bad. Driving mistake can have deadly consequences, because the road is small and goes along deep valleys. No barrier stands between the road and the cliffs.
The Pamir Highway has very little traffic, but for that reason nearly everybody stops. A small payment can be expected, make it clear before you enter. Russian is a common language. You need special permits for some regions along the Pamir highway, which must be cleared before you enter. If you apply for a visa for Tajikistan, ask for the permits at the same time. Be aware of landmines down in the Panj River.
The supply with fresh vegetables and fruits in the villages on the road is bad. You will most get conserved food. As Anton Krotov says, Tajik people are very hospitable. There is almost no grocery shops from Kulob to Sarytash except a little bazaar in Khorog. This doesn't mean that you need to have backpack full of food. When you was picked up by locals they will feed you all the way. Please, do not refuse, let people from Pamirs show how they are hospitable.
Please keep patience, relax and be ready to wait for a days in some places. The number of cars is tended to zero on part from Kalaikhumb to Sarytash. From Kulob to Khorug up to 50 in a day in both directions, mostly heavy trucks and fullfilled taxi. From Khorug to Murgab up to 25 in a day in both directions. From Murgab to SaryTash up to 5 cars in a day. There is a truck with gasoline from Murgab to Tajik border control in Sarytash direction and back 2-3 times in a week. Also there is 23 km of no mans land between Tajik and Kyrgyz control points. Be ready to walk this distance through mountain pass Kyzylart 4200 meters altitude. Best option to start hitchhiking through Pamir track is to start from Osh, then move to Sarytash and then move to Murgab. This direction gives advantage of new transit cars (buses, trucks, tractors etc.) going from China to Tajikistan. There could be up to 50 transit cars in a day in Sarytash>>>Murgab direction. Most trucks on Pamir track transport cargo from China, so before going to Pamirs find out schedule of holidays on Tajik and Chinese border controls. In some periods of year China got up to 10 days holidays, so up to 1 week there will be no cargo trucks on Pamirs.
Probably traffic is higher in high season when tourist transit the road with offroad private cars, so better traffic in July/August. Don't hitchhike a truck if the road is unpaved, steep or in bad condition unless you don't want to get stuck at 15 km/h for hundreds of km. Very difficult to estimate an average km/day because depends if you are in a offroad car or in a truck and depends on road condition (paved/unpaved/steep). For me was average 300km/day hitchhiking only offroad cars in daylight.
From Osh to Sary Tash: a lot of traffic both cars and trucks, paved road in good condition, average speed 80 km/h. From Sary Tash to Murghab: just a few private offroad cars, most of them full of people, good road condition both paved and unpaved, average speed 80 km/h on offroad car. Altitude from 3200m to over 4000m, highest peak 4650m. Temperature: Murghab 3600m max 19C min 7C in July. From Murghab to Khorogh: a lot of offroad cars, most of them full of people, few trucks (Shacman chinese brand), first 150 km unpaved road in bad condition 40 km/h with offroad car, 20 km/h with truck, last 150 km mostly paved road 80 km/h with offroad car. Altitude: From 3600m to over 4000m down to 2000m. From Khorogh to Qal'ai Khumb: a lot of offroad cars, most of them full of people, few trucks (Shacman chinese brand and Kamaz old russian), unpaved road in bad condition, 40 km/h with offroad car, 20 km/h with a truck. From Qal'ai Khumb to Dushanbe: most of the traffic directed to Dushanbe leave the M41 to take the road to Kulob. This road is paved and in good condition, 80 km/h in a car if is not too steep.