Hiroshima is a city in Japan.
Service Areas and Parking Areas are a great place to start hitchhiking and luckily enough, those around Hiroshima can easily be accessed through local public transportation. To see the map of all the SA/PA, you can use Nexco. Here is a map of Hiroshima surroundings.
There are at least one PA and two SA you can access from Hiroshima. They can be used for both going East or West.
The easiest and cheapest, only 400 yens by train, is Numata PA. GPS coordinates are 34.464445, 132.422462. You can find public transportation itineary with Google maps or follow instructions below. To access this PA, take the Astram line and stop at Tomo Station. From this station, take South exit, turn right and take your first left. Go down towards the river, cross the bridge on your right and continue straight. Cross the main road and keep going straight until you reach a junction with a parking on your left and take right towards the Big House store. Then take first left. At the main road, take the street across, which is slightly on your right. Go up until you see a parking. If you want to go east, towards Kobe, Osaka, Kyoto, ...then take left and a couple of meters later, you will see stairs to access the PA. If you are going west, continue under the tunnel and you will find stairs on your left right after.
The other options are SA, that have probably more traffic but are more expensive to reach from Hiroshima.
Miyajima SA is a good option if you come from Miyajima island. Kodani SA is pretty big but quite far.
Experience summer 2018 Numata PA, going East toward Kyoto: Got a ride within 20 minutes asking drivers around if they could take me. Most said no but eventually one said yes and brought me to the next SA. Good spot, but the traffic was quite light although it had a LOT of trucks (less regular cars, but many TRUCKS). Very good spot nonetheless and accessible. Just be friendly with the staff to make sure they don't kick you out.