Just a few things you should consider in your appeal.
Firstly - you will need to have made a submission or observation on the original application to the county/city council, in order to have the right to object/appeal to the Board. If this is not the case - find someone who has made a submission - and speak nicely to them. You can ask for leave to appeal to the Board if you have no objected, but this is only in special circumstances.
Secondly, go to the local authority and ask for a copy of the decision, the planner's report, and any reports done by the departments of the local authority. These will all be on the file and they will photocopy them for you for a small charge. I imagine if traffic is an issue the Roads Department will have highlighted any particular concerns about traffic and access - if indeed it is an issue with the site. As a matter of course get all the reports on file as they could provide you with good 'amunition' for the appeal (if not on traffic grounds then maybe sewerage/sanitary services etc.)
Lastly, as the previous poster recomended, look to the Development Plan and the Residential Density Guidelines for the local and national guidance on residential development. In terms of the Dev. Plan, issues such as zoning, density, plot ratio, site coverage, overlooking, overshadowing, insufficient/poor quality open space, traffic, impact on residential amenity (existing), car parking standards, are the important sections to reference. - you can usually find something that you can criticise them on. If anything is particularly relevant the Planner should have picked it up in their report.
NB - check with the Board on the last date of appeal... there is no flexibility on this.