Not necessarily. In many states, you can drive in whichever lane you wish, just as long as you 
stay in that lane and not switch around. That way, the traffic can stay in the right lane and just cruise on by you.
Of course, this is not appropriate on 2-lane roads. 
		 
		
	 
ok  here is the list 
StateKeep  Right?LawComments Alabama        slower
32-5A-80(b)
Alaska        <SL
13  AAC 002.50(b)Keep right if below speed limit. Arizona        slower
28-721(B)
Arkansas    no27-51-301(b)     Law prohibits obstructing traffic by driving  continuously in the left lane. California    slower
VC  21654(a)"Notwithstanding the prima facie  speed limits" Colorado    slower*             42-4-1001(2),             42-4-1103(3),             
42-4-1013          The left lane is reserved for passing when the speed limit     is 65 or higher.     
State  Patrol brochure discussing law Connecticut    slower
14-230(b)Passing  on right on Interstate prohibited when only two lanes, 
14-233(4) Delaware    slower
21-4114(b)
D.C.        slower
18-2201.2
Florida        slower
316.081(2)Governor  Bush vetoed 2005 
SB732,  which would have reserved the left lane for passing, saying that  drivers blocking the left lane are "cautious and careful." Georgia        slower*          
40-6-40(b),     
40-6-184(a)(2)     If below speed limit in left lane and blocking  overtaking traffic, must move right. Hawaii        slower
291C-41(b)Honolulu  prohibits driving more than 5 MPH under the limit in the left lane. Idaho        slower
49-630(2)
Illinois    yes
625  ILCS 5/11-701(b),(d)Keep right except to pass on limited  access highways effective January 1, 2004. Indiana        slower
9-21-8-2(b)
Iowa        slower321.297(2)
Kansas        yes8-1522(c), 8-1514(b)Keep  right law effective July 1, 2009 (
bill  154 of 2009 session) Kentucky    yes
189.340(7)Only where the speed limit is 65 or 70 Louisiana    yes
R.S. 32:71On multilane highways keep right except to pass and move  right if blocking overtaking traffic. Maine        yes
29A-2052(6)Only where the speed limit is 65 Maryland    <SL
21-301     If driving 10 MPH under speed limit, or slower  than speed of traffic if conditions require speed below limit.          
SB 701  would require vehicles in the left lane to give way to faster traffic. Massachusetts    yes
89-4BPassing  on right prohibited on undivided two-way road, 
89-2 Michigan    weird
257.634Except in heavy traffic or on freeways with three or more  lanes. Minnesota    slower
169.18(10)
Mississippi    slower
63-3-603(d)
Missouri    slower
304.015(3)     304.151  prohibits "obstruct[ing] the regular flow of traffic on...any state  highway" Montana        slower61-8-321(2)
Nebraska    slower
60-6,131(2)
Nevada        slow     
484.373Move  right if "imped[ing] ... movement of traffic" New Hampshire    slower
265:16
New Jersey    yes39:4-88Passing  on right prohibited unless vehicles are in "substantially continuous  lines".  39:4-85. New Mexico    slower66-7-308(B)
New York    slowerV&TL 1120(No  link -- New York hides its laws from people using text browsers.) North Carolina    <SL
20-146(b),(e)Keep  right if below speed limit. North Dakota    slower
Ohio        <SL
4511.25Ohio used  to follow the UVC.  In 2006 bicyclists lobbied for and won the right to  obstruct traffic by going the speed limit.  Left lane for passing only  on Turnpike, regulation 
5537-2-09. Oklahoma    yes
47-11-301,  
47-11-309Keep  right except to pass on four lanes in effect late 2009. Oregon        slower
811.315
Pennsylvania    usually75-3313(d),  75-3301(b)     May also use left lane to allow traffic to merge  or "when traveling at a speed greater than the traffic flow".   Puerto Rico    <SL
Title  9 §5123
Rhode Island    slower
31-15-2No  passing on right on two-way street, 
31-15-5 South Carolina    slower
56-5-1810(b)
South Dakota    no
32-26-1"Slow moving vehicles" keep right Tennessee    slower55-8-115(b)
Texas        slower
545.051(b)DOT  may post "left lane for passing only", 
544.011.   Passing on right prohibited except on one-way roadways, 
545.057. Utah        slower*
41-6a-701(3),  
41-6a-704Must  move right to let faster traffic pass. Vermont        slower23-1031(b)
Virginia    slower*
46.2-804(1)46.2-842.1  requires vehicles in the left lane to yield to faster traffic; 
State  Police say this applies even when faster traffic is speeding. Washington    usually
46.61.100(2)May also use left lane to allow traffic to merge or "when  traveling at a speed greater than the traffic flow". West Virginia    slower
17C-7-1(b)
Wisconsin    slower346.05(3)
Wyoming        slower
31-5-201(b)Law excepts "one-way streets".  Effective July 1, 2005, it  is illegal to block traffic moving within the speed limit by driving in  the left lane for a long time.   The Uniform Vehicle Code states: 
Upon all roadways any vehicle proceeding at less than the normal speed of traffic at the time and place and under the conditions then existing shall be driven in the right-hand lane then available for traffic ... 
Note that this law refers to the "normal" speed of traffic, not the "legal" speed of traffic.  The 60 MPH driver in a 55 MPH zone where everybody else is going 65 MPH must move right.  Contrast Alaska's rule, 13 AAC 002.50, allowing vehicles driving at the speed limit to use the left lane, and Colorado rev. stat. 42-4-1103, prohibiting blocking the "normal and reasonable" movement of traffic. 
 (Enforcement for failing to keep right while at or above the speed limit is variable. Toledo police have ticketed truck drivers for driving at the 60 MPH speed limit in the left lane.  Police looking for criminal activity are aware of the "keep right" law and will use it as an excuse to stop a suspicious car.  On the other hand, a New York judge announced that he would not convict drivers for blocking speeding traffic, People v. Ilieveski, 175 Misc. 2d 943; 670 N.Y.S.2d 1004  (Monroe County N.Y. 1998), and most police find speeding easier and more profitable to enforce.)
it didnt paste in correctly but here is the link
http://www.mit.edu/~jfc/right.html
tom