George H. Bush Intercontinental Airport (IAH) is Houston's primary airport and one of the largest international airports in America in terms of size and number of flights. If driving, JFK Blvd leads you to an Airport Tollway for the quickest route to Downtown.
 
William Hobby Airport (HOU) is a regional airport that has been modernized and expanded to become a secondary airport with many national flight connections. If you are traveling to South, Southwest or Southeast Houston, this may be your best time saving choice to/from the airport. If you don't know the local streets well, I-45 freeway should be your only option leaving HOU airport.

Car Rentals: Take the Rental Car Shuttle for Avis, Hertz, National and Thrifty car renters who are on or near both airports.

Taxis: Houston has the third-largest taxi fleet in the country and four patrons can ride for the price of one in all taxis. You can always get a taxi at hotels Downtown, Galleria/Post Oak, Texas Medical Center and Greenway Plaza. From IAH airport to Downtown costs about $40. HOU Airport is a better option for shorter taxi rides to Downtown, Texas Medical Center, Greenway Plaza and Galveston. Taxis include:

Fiesta Cab    713-236-9400
Liberty Cab   713-695-6700
United Cab    713-699-0000
Yellow Cab    713-236-1111

Amtrak Sunset Limited travels from Los Angeles to Phoenix to San Antonio to Houston to New Orleans to Mobile to Jacksonville. The train station is at 902 Washington Avenue, downtown (MAP). Surprisingly, there is no Amtrak service from Houston to Dallas.
  

Houston METRORail is currently a single light rail line connecting Downtown to Midtown to Museum District to Texas Medical Center to Reliant Stadium area. Though very late compared to other major cities, Houston is developing a large METRORAIL and commuter rail network implemented in phases between 2011-2020. A FREE Downtown trolley connects with METRORail and stops at popular destinations such as the Theater District, Bayou Place, Main Street Square, Shops at Houston Place, Minute Maid Park, Discovery Green and Toyota Center.

Houston has about 14 Transit Centers with parking and easy commuter transfers for Bus Rapid Transit.

Houston Freeway & Tollway Network: Immense and sprawling, Houston freeways and tollways take advantage of modern design architecture, HOV lanes, metered entrances, and freeway status signs, enabling them to transport more vehicles per lane. Unfortunately, HOV lanes are heavily used by carpools and vanpools, so don’t get your hopes up with this mode of transportation.

The Hardy Tollway is recommended during commute hours from the I-610 Freeway interchange to IAH airport. I-610 Loop, I-10 and I-45 freeways are crush packed during normal commute hours. The Good News - you can also traverse Harris County quickly via the Sam Houston Tollway by circling Houston 12 miles outside Downtown.

Residents refer to freeways and tollways by name or number:

      Hardy Toll Road
    8 Sam Houston Tollway
  10 Baytown East Freeway/Katy Freeway
  45 North/Gulf Freeway
  59 Eastex/Southwest Freeway
  90 Crosby Highway
225 Pasadena Freeway
288 South Freeway
290 Northwest Freeway
610 East Loop/North Loop/ West Loop/South Loop