Phnom Penh is the capital city of Cambodia.
South towards Kampot, Sihanoukville, Koh Kong
There is a roundabout called Chaom Chau (វង្វង់មូលចោមចៅ) [11.5344N, 104.8296E], 12 km from the city center or 2.6 km from the airport entrance. This is where the national roads NH3 (south) to Kampot and NH4 (west) to Sihanoukville start. This makes a good hitchhiking spot. A tuk-tuk from the train station will be between 2 US$ and 6 US$, depending on the time and how good you haggle. Public bus number 3 goes to the roundabout beginning at the Night Market, and Public Bus number 4B (but NOT 4A) goes to the roundabout through Chroy Changvar Bridge, Wat Phnom, Central Market, (north of) Orussey Market, and Daeum Kou Market following St.217, and goes along the NH4 (get off at the last stop to get a better hitchhiking spot to Sihanoukville or Koh Kong).
East towards Ho Chi Minh City
Take bus number one to the last stop Okhna Souy Sophan Bus Terminal. Continue following highway number one.
Can take a cheap minivan to the Tonloap Market (ផ្សារទន្លាប់), on the border with Vietnam, at the Caltex Petrol Station of Daeum Kou Market [11.55285N, 104.9037E] or at the Sokimex Petrol Station of Steung Mean Chey [11.5348N, 104.8857E] (which you can take Public Bus number 4B to both). Otherwise, head south on Chamkar Doung Road towards the Killing Fields (ជើងឯក) from the Sokimex Petrol Station, and hitchhike from there.
North towards Siem Reap, Laos, Kratie, Mondulkiri, Ratanakiri
Beyond the four-faced statue across the Chroy Changvar bridge [11.5894N, 104.9271E] following the NH6 is good for hitchhiking.
Northwest towards Udong, Kampong Chhnang, Pursat, Battambang, Poi Pet, Bangkok
Take the Public Bus Line 1 as far north as it goes [11.63527N, 104.8868E], or alternatively take a remorque just north of Chroy Changvar bridge [approx. 11.5872N, 104.91914E], which usually go as far as Udong (about 40km away).
Since late 2014 Phnom Penh now has a very small bus network of three routes. The most interesting is line 3 along Confederation de la Russie between the night market by the river in the east and past the airport to the Choam Chao roundabout.The bus is air-conditioned, comfortable, and not crowded. Tickets are KHR 1,500. Otherwise you can use Tuk-Tuks or motorbike-taxis to leave the city.