[mhagain]

Originally Posted by The Mouth of Sauron

When the 4 hobbits left Tom Bombadil's house to make their way to the Road, why didn't they just ride north along the eastern edge of the Old Forest rather than getting tangled up in the Barrow Downs?

They did.

They rode off along a path that wound away from behind the house, and went slanting up towards the north end of the hill-brow under which it sheltered ..... Goldberry spoke to them and recalled their eyes and thoughts. 'Speed now, fair guests!' she said. 'And hold to your purpose! North with the wind in the left eye and a blessing on your footsteps!'

And:

They rode across and looked northwards. Then their hearts rose, for it seemed plain that they had come further already than they had expected. Certainly the distances had now all become hazy and deceptive, but there could be no doubt that the Downs were coming to an end. A long valley lay below them winding away northwards, until it came to an opening between two steep shoulders. Beyond, there seemed to be no more hills. Due north they faintly glimpsed a long dark line. That is a line of trees,' said Merry, 'and that must mark the Road. All along it for many leagues east of the Bridge there are trees growing. Some say they were planted in the old days.'
'Splendid!' said Frodo. 'If we make as good going this afternoon as we have done this morning, we shall have left the Downs before the Sun sets...

The Downs themselves seem to have come right to the eastern edge of the Old Forest, so by riding along the eastern edge of the Forest one is in the western edge of the Downs, but it was only when the fog came down that things really got messed up.