There are several roads leading to Thingvellir National Park which is located approximately 45 km northeast of Reykjavík.
Take road no. 1 heading north out of Reykjavík. After driving through the town Mosfellsbær take the first exit to the right at roundabout onto road no. 36 to Thingvellir. This route follows the main road to Thingvellir and is ploughed most days during the winter.
Another option in summertime out of Reykjavik is to take road 1 towards Hveragerdi/Selfoss on the south coast. Just after leaving the Reykjavik city limits turn left onto road 431 and follow it onto road 435 (Nesjavallaleid). This road will follow the pipeline delivering hot water to Reykjavik from the Nesjavellir power plant in the Mt. Hengill area. The road crosses the Hengill volcano providing great views of Lake Thingvallavatn coming down from the mountain on the north side. When coming down the mountain turn left on road 360 (Grafningsvegur) and drive about 11 km along the banks of Lake Thingvallavatn until turning right onto road 36. Follow that about 8 km until reaching the visitor center at Thingvellir National Park at Hakid (Fraedslumidstod).
Note that this road (435) is closed in winter and only open from May-September depending on snow conditions. Make sure to check conditions before driving it.
From Geysir and Gullfoss.
Take road 35 onto road 37 to Laugarvatn. At the roundabout on the outskirts of the town of Laugarvatn take the first right exit onto road 365 followed by road 36 to Thingvellir..
From the Borgarfjordur area in the west.
There are two options which are only open in the summertime and some restrictions might apply for rental cars.
Driving road 50 in Borgarfjordur turn on to 52 in Lundarreykjardalur valley and drive east and turn right on to road 550. From Husafell take road 550 over Kaldidalur to Thingvellir.
The coordinates for Thingvellir (Hakid viewpoint) are : 64° 15,327'N, 21° 7,691'W
Make sure to check road conditions before driving in Iceland on the Road Administration maps.
For further information on safe travel in Iceland visit Safe travel.