The road from Yudanaka to the Monkey Park is very narrow, but totally drivable in a dry summer day (closed in the winter time).
Drive slowly, watch the upcoming traffic (mirrors placed on every curve help a lot) and it is perfectly safe.
On the end of the road will be paid parking lot (around $5) and little refreshment stand.
Park your car, put some UV sun screen on you, good shoes, hat and wet towel and head following the signs to the Monkey Park.
After hiking for about 15 minutes (easy trail, full of senior citizens and little children, but no strollers) you will pass the Koraku Kan onsen and you will arrive at the entrance of the park, where you will purchase your entrance ticket ($5).
Right after passing the ticket booth you will be surrounded by monkeys. They will be everywhere. Some will not pay attention to you, some will watch you just as much as you will watch them.
One thing for sure...despite overwhelming stench and millions of jumping fleas, you will have a great time and would want to stay there and stare at the monkeys for entire day, so bring plenty of cold water with you to drink and wet your towel.
The road from Yodanaka town to Jigokudani Park is narrow and windy but perfectly safe if you are driving slow and watching the mirrors located on every curve.
Here are some pictures from our ride.