This topic caught my attention so I thought I would do some research. I tracked down the laws on left lane restrictions and speed limits. I can't find an exact definition but I have come to the conclusion that "auto stage" is commercial transportation, such as a taxi.

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WAC 468-510-020 Left lane restrictions.(1) RCW 46.61.100(3) mandates that no vehicle towing a trailer or no vehicle or combination over 10,000 lb. may use the left lane of limited access roadways having three or more lanes in one direction, and that a high occupancy vehicle (HOV) lane is not considered the left hand lane of a roadway. Within this section, 10,000 lb. means 10,000 lb. gross vehicle weight (G.V.W.). (2) RCW 46.61.100(3) further mandates that the department, in consultation with the Washington state patrol, shall adopt rules specifying those circumstances where it is permissible for other vehicles to use the left lane in case of emergency or to facilitate the orderly flow of traffic, and those segments of limited access highways exempt from the subsection due to the operational characteristics of the roadway.
(a) For the types of vehicles specified, and under the circumstances enumerated in (a)(i) through (vii) of this subsection, the left lane prohibition described in subsection (1) of this section does not apply to:
(i) Motorcycles towing trailers.
(ii) Class B motor homes, commonly called conversion vans, without a motor vehicle or trailer in tow.
(iii) Tow trucks weighing over 10,000 lb. G.V.W. when en route to an emergency on a specific roadway or roadside.
(iv) Fire trucks or emergency care vehicles weighing over 10,000 lb. G.V.W. when en route to an emergency.
(v) Any vehicle towing a trailer or vehicle or combination weighing over 10,000 lb. G.V.W. when one or more of the lanes are blocked because of an accident, other incident, or highway maintenance or construction activities.
(vi) Any vehicle authorized to use a HOV lane that would otherwise be prohibited from the left lane within two miles approaching the beginning of a HOV lane or following the terminus of a HOV lane.
(vii) Any department of transportation vehicle towing a trailer or weighing over 10,000 lb. G.V.W. when conducting official business within the left lane.
(b) On the roadway portions enumerated in (b)(i) through (viii) of this subsection, the left lane prohibition described in subsection (1) of this sectiondoes not apply:
(i) On northbound and southbound Interstate 5 in the Vancouver vicinity, from the Washington/Oregon state line to exit 3 at Main Street.
(ii) On northbound Interstate 5 in the Vancouver vicinity, from the confluence of Interstate 205 to exit 9 at 179th Street.
(iii) On southbound Interstate 5 in the Vancouver vicinity, from exit 9 at 179th Street to exit 7 at Interstate 205.
(iv) On northbound Interstate 5 in the Seattle/Everett vicinity, from exit 154A at I-405 to exit 194 at SR 529.
(v) On southbound Interstate 5 in the Seattle/Everett vicinity, from exit 189 at SR 526 to exit 154A at I-405.
(vi) On eastbound and westbound Interstate 90 in the Seattle vicinity, from exit 2A and 2B respectively at Interstate 5 to exit 10A at Interstate 405.
(vii) On eastbound and westbound Interstate 182 in the Tri-cities vicinity, from exit 4 to exit 12A.
(viii) On northbound and southbound Interstate 205 in the Vancouver vicinity, from the Washington/Oregon state line to the termini of the three lane sections about one-half mile south of exit 32.
(c) On the roadway portions enumerated in (c)(i) and (ii) of this subsection, the left lane prohibition described in subsection (1) of this section does not apply to any vehicle, except trucks over 10,000 lb. G.V.W., when using the left lane for passing to facilitate the orderly flow of traffic:
(i) On southbound Interstate 5 in the Southcenter vicinity, from exit 154A at I-405 to exit 151 at South 200th Street.
(ii) On southbound Interstate 5 in the Tacoma vicinity, from exit 135 at SR 167 to exit 130 at South 56th Street.

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RCW 46.61.410
Increases by secretary of transportation -- Maximum speed limit for trucks -- Auto stages -- Signs and notices.

(1)(a) Subject to subsection (2) of this section the secretary may increase the maximum speed limit on any highway or portion thereof to not more than seventy miles per hour in accordance with the design speed thereof (taking into account all safety elements included therein), or whenever the secretary determines upon the basis of an engineering and traffic investigation that such greater speed is reasonable and safe under the circumstances existing on such part of the highway.

(b) The greater maximum limit established under (a) of this subsection shall be effective when appropriate signs giving notice thereof are erected, or if a maximum limit is established for auto stages which is lower than the limit for automobiles, the auto stage speed limit shall become effective thirty days after written notice thereof is mailed in the manner provided in subsection (4) of this section.

(c) Such maximum speed limit may be declared to be effective at all times or at such times as are indicated upon said signs or in the case of auto stages, as indicated in said written notice; and differing limits may be established for different times of day, different types of vehicles, varying weather conditions, and other factors bearing on safe speeds, which shall be effective when posted upon appropriate fixed or variable signs or if a maximum limit is established for auto stages which is lower than the limit for automobiles, the auto stage speed limit shall become effective thirty days after written notice thereof is mailed in the manner provided in subsection (4) of this section.

(2) The maximum speed limit for vehicles over ten thousand pounds gross weight and vehicles in combination except auto stages shall not exceed sixty miles per hour and may be established at a lower limit by the secretary as provided in RCW 46.61.405.

(3) The word "trucks" used by the department on signs giving notice of maximum speed limits means vehicles over ten thousand pounds gross weight and all vehicles in combination except auto stages.

(4) Whenever the secretary establishes maximum speed limits for auto stages lower than the maximum limits for automobiles, the secretary shall cause to be mailed notice thereof to each auto transportation company holding a certificate of public convenience and necessity issued by the Washington utilities and transportation commission. The notice shall be mailed to the chief place of business within the state of Washington of each auto transportation company or if none then its chief place of business without the state of Washington.

[1996 c 52 § 1; 1987 c 397 § 4; 1977 ex.s. c 151 § 35; 1974 ex.s. c 103 § 2; 1970 ex.s. c 100 § 1; 1969 ex.s. c
12 § 1; 1965 ex.s. c 155 § 55; 1963 c 16 § 3. Formerly RCW 46.48.013.]

NOTES:

Intent -- 1987 c 397: "It is the intent of the legislature to increase the speed limit to
sixty-five miles per hour on those portions of the rural interstate highway system where the increase would be safe and reasonable and is allowed by federal law. It is also the intent of the legislature that the sixty-five miles per hour speed limit be strictly enforced." [1987 c 397 § 1.]