"The right of the citizen to travel upon the public highways and to transport his property thereon, either by carriage or by automobile, is not a mere privilege which a city may prohibit or permit at will, but a common law right which he has under the right to life, liberty, and the pursuit of happiness".
The key difference between the 2 cases is the purpose of the statute / ordinance involved & I think it's safe to say that Maryland doesn't have the law addressed in Hendrick v. Maryland anymore. A key issue in that case, understandable given the timeframe (1910-1915) when automobile / truck use was 1st growing by leaps & bounds, was how to raise revenue to build new roads & maintain the ones already there, which had been designed for horse & buggy, not cars & trucks.
Regarding the motor vehicle law in Hendrick, note the following:
"Other sections relate to speed, rules of the road, accidents, signals, penalties, arrests, trials, fines, etc. All money collected under the provisions of the Act go to the commissioner, and except so much as is necessary for salaries and expenses must be paid into the state treasury to be used in the construction, maintaining, and repairing the streets of Baltimore and roads built or aided by a county or the State itself." (Emphasis added).
The court clearly determined that the purpose of the Act was fine, but since then, the States & the federal Gov't. enacted "highway use" taxes which are applied to gasoline & diesel fuel @ the pump, so no special registration, plate, etc. such as Maryland required in 1910 is necessary. The more you drive, the more fuel you buy & the more highway use tax you pay. I think that's a pretty fair measure & if you don't drive @ all, you don't pay the tax. Although, both the State & federal governments are crying now because newer vehicles get much better fuel mileage, so you can drive the same distance you always have & pay less highway use tax because you don't need to buy as much fuel as you used to.
The Thompson v. Smith case was a different ballgame & I'm going by memory here. The full text of that decision is not on the internet & years ago I photocopied it out of the Southeastern Reporter in a university law library, but I'm not going to take the time to find my copy now. If I remember correctly, a certain small city in Georgia had passed an ordinance requiring a driver's license just to drive in or thru that city, based on the concept that it's a "privilege" to use the public roads. And it is if you use the roads in commerce - but it's not if you're using the roads on your own personal business. In other words, I drove a taxi for a while when I went to college (since I could choose my own working hours) & I drove semis cross-country for several years - both a taxi & a semi are "commercial vehicles for hire" operated in an activity which can be licensed, regulated & taxed. But I don't need a license to use my "personal conveyance" to go to the grocery store.
Along the same lines, back when I was driving semis, Ohio got slam-dunked by the feds for using a rest area as a weigh station & commercial vehicle checkpoint. Funny little story about that - I drove right past it, but didn't see the temporary sign drivers were talking about on the CB radio, which said something like "All commercial vehicles must exit", with an arrow pointing to the entrance of a rest area. The reason I didn't see the sign, from what I heard on my radio, was that drivers kept knocking it down with their huge front bumpers & by the time I got there, there wasn't enough left of the sign to put back together!
But I digress - I followed up on that situation later & the federal Gov't. said that the rest areas along interstate highways are "for the convenience of the traveling public" & the State had no authority to use one as a temporary weigh station (some state troopers carry portable scales), or to check semi drivers' log books, licenses, etc. The State of Ohio got a huge fine over that & was ordered to never do it again. And if you need more weigh stations, build 'em yourself - commercial vehicles & the "traveling public" are 2 different things.
From the case law I've read, it appears that it was in the 1930's when the line between commercial vehicle for hire & "personal conveyance" started to get blurred - in Thompson (around 1930), the city-issued license ordinance was held to be invalid & several States did not have driver's license laws like those since then & especially since WWII. But note that the federal definition of "motor vehicle" still is commercial vehicle:
18 U.S.C. § 31 Definitions:
(a) (6) Motor vehicle.— The term "motor vehicle" means every description of carriage or other contrivance propelled or drawn by mechanical power and used for commercial purposes on the highways in the transportation of passengers, passengers and property, or property or cargo. (Emphasis added).
Compare that with NC's definition:
N.C.G.S. § 20-4.01 Definitions.
(23) Motor Vehicle. – Every vehicle which is self-propelled and every vehicle designed to run upon the highways which is pulled by a self-propelled vehicle. This shall not include mopeds as defined in G.S. 20-4.01(27)d1.
Now, do the States have the right to re-define the term "motor vehicle"? In our society where drivers (commercial & private) cross State lines all the time, I would say no. Another reason I say no deals with the Constitutional ideal that States must honor each other's laws & an example of a guy from Arizona who got stopped in NC for having the windows on his car tinted too dark. But the dark tint was legal in Arizona & the car had AZ tags, so the charge was dismissed.
And as much as I hate the federal Gov't. setting standards, when it comes to interstate travel, I don't have a problem with it. For example, when I drove semis in the late '70's & early '80's, the States had different weight / length laws & while you could be legal in the State you picked up a load in & the State where you were going to deliver it, you'd often be overweight or over length in States you had to go thru on the way. But since then, the weight / length rules have been standardized nationwide.
"Motor vehicle" is actually a misnomer (except for electric vehicles), but no matter how many definitions the States come up with, at least the federal Gov't. recognizes to some degree the difference between a commercial vehicle which is a "privilege" to operate & "personal conveyance" which you have a right to operate. There's a ton of case law on all this too, but it is not "standardized" by any stretch of the imagination. But go back to the 1930's & earlier - driver's licenses then were for the "privilege" of operating a commercial vehicle for hire.
See also, Frost & Frost Trucking v. R.R. Comm'n of California, 271 U.S. 583 (1926):
1. Assuming that the use of its highways by private carriers for hire is a privilege which the state may deny, it cannot constitutionally affix to that privilege the unconstitutional condition precedent that the carrier shall assume against his will the burdens and duties of a common carrier. P. 271 U.S. 592. (Emphasis added).
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There is involved in the inquiry not a single power, but two distinct powers. One of these, the power to prohibit the use of the public highways in proper cases, the state possesses, and the other, the power to compel a private carrier to assume against his will the duties and burdens of a common carrier, the state does not possess.
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It would be a palpable incongruity to strike down an act of state legislation which, by words of express divestment, seeks to strip the citizen of rights guaranteed by the federal Constitution, but to uphold an act by which the same result is accomplished under the guise of a surrender of a right in exchange for a valuable privilege which the state threatens otherwise to withhold. It is not necessary to challenge the proposition that, as a general rule, the state, having power to deny a privilege altogether, may grant it upon such conditions as it sees fit to impose. But the power of the state in that respect is not unlimited, and one of the limitations is that it may not impose conditions which require the relinquishment of constitutional rights. If the state may compel the surrender of one constitutional right as a condition of its favor, it may, in like manner, compel a surrender of all. It is inconceivable that guaranties embedded in the Constitution of the United States may thus be manipulated out of existence.