I would like to comment about challenges in VANET

1. Irregular traffic and discontinuous connectivity

2. Limited sensing capability of nodes (vehicles)

3. Position of sink

Probably the following threats and attacks are the main security issues and challenges

Denial of Service Attack, Malware, Black Hole Attack, Sybil Attack, Replay Attack, GPS Spoofing, Key Replication, Spamming etc.

Access control is one of the biggest challenges: Who has the right to operate in the VANET and also who will be in charge to regulate and coordinate all of the users.

Synchronization is biggest problem in VANET 

Thank you all.. I appreciate your comments.