As Ko Samet is an island, you first have to drive to Rayong. From Bangkok, you can take Sukhumvit Rd (Highway No. 3) passing Chonburi, Si Racha, Pattaya, Sattahip and onto Rayong. The total distance to Rayong is approximately 220 kilometers. If you drive onto Highway No. 36 at Bang Lamung (before Pattaya), you'll take a shortcut inland and save about 45 kilometers (but the scenery is not as impressive). There is also less traffic.