It connects Panvel, a city east of Mumbai (Bombay) to Cape Comorin (Kanyakumari) via Mangalore, passing through the states of Maharashtra, Goa, Karnataka, Kerala and Tamil Nadu.

Owing to higher density of population and high land value, national highway will be 45 meter width, 6 lane, in Kerala.

The National Highway 66 (previously numbered NH-17) connects cities, towns and major villages of different states as follows: Panvel, Pen, Mangaon, Mahad, Poladpur, Khed, Chiplun, Sangameshwar, Ratnagiri, Lanja, Rajapur, Kankavli, Kudal, Sawantwadi.

Karwar, Ankola, Kumta, Honnavar, Manki, Murudeshwar, Bhatkal, Shiroor, Baindur, Uppunda, Navunda, Maravanthe, Hemmadi, Tallur, Kundapura, Koteshwar, Kota, Saligrama, Brahmavar, Udupi, Kapu, Padubidri, Mulki, Surathkal, Mangaluru, Thokottu, Ullal, Kotekar, Talapady.

Uppala, Kasaragod, Kanhangad, Payyannur Pariyaram, Taliparamba, Dharmashala, Kannur, Dharmadam, Thalassery, Mahe, Vatakara, Payyoli, Koyilandi, Kozhikode, Ramanattukara, Tenhipalam, Kottakkal, Puthanathani, Valanchery, Kuttippuram, Thavanur, Ponnani, Chavakkad, Vadanappally, Kodungallur, Moothakunnam, North Paravur, Koonammavu, Varapuzha, Cheranallur, Edappally, Kochi, Aroor, Cherthala, Alappuzha, Ambalapuzha, Haripad, Kayamkulam, Karunagappally, Chavara, Neendakara, Kollam, Mevaram, Kottiyam, Chathannoor, Kallambalam, Attingal, Kazhakkoottam, Thiruvananthapuram, Balaramapuram, Neyyattinkara, Parassala.

Kerala and Goa have requested a narrower width of 45 m (148 ft), and there are protests against the highway being widened in these states.

But the high value of land will make it hard for compensation, and the ruling party's influence is controversial in relation to similar acquisitions like GAIL pipeline.

[11][12] Now the process is going fast in spite of protests from various highway action forum groups,[13] especially because it connects Kerala's capital Thiruvananthapuram to Kochi and of tourism importance.

Due to narrow span and lack of dividers, there are no separate lanes for oncoming traffic, driving and passing (over-taking).

On 19 March 2013 a bus carrying over 50 persons fell off a bridge close to a river in the Khed area of Ratnagiri district.

Hence this National Highway 66 allows for the movement of goods by trucks (lorries) from the hinterland to the major sea ports of India.

Many industries have been set up along National Highway 66, taking advantage of connectivity and proximity to ports on the Arabian Sea.

NH 66 passes through hills, forests, rivers, rivulets, and streams, generally flowing westwards towards the Arabian Sea.

The road is uneven, and bumps, curves, steep rises and narrow paths in between hillocks are found all over this highway.

This highway is expected to lose importance in coastal Karnataka as the Ministry of Road Transport & Highways, Government of India has proposed a greenfield (i.e., new and parallel) access controlled expressway corridor connecting the port cities of Mangaluru, Karwar, and Panaji.

(The 3D right-of-way concept means adding the space (height) element to the usual length and breadth dimensions involved in road construction.

)[citation needed] • A road trip along the highway was made popular by the Bollywood films Bombay to Goa and Dil Chahta Hai.