It's not a bad analogy.
In fact cars, roads and speed restrictions are a great analogy for the whole network / speed issue.
You can buy any car, with any performance spec, from nearly anyone and use it on a variety of roads with different conditions. These parameters leads to a large number of possible permutations.
1. Cars - You can buy a car for as little as a few thousand rand (clearly not new and it might have serious problems) to decent new cars for around R100K to 'typical' cars for closer to R300K, right up to hyper-cars costing a few million. The cars will have power output that vary from less than 50KW all the way up to hundreds of KW. Top speeds can range from 80Km/h to 440Km/h.
Yet all these cars can be driven on the road. Importantly, car manufacturers don't build roads.
2. Roads - We have from toll roads with many lanes and safe up to high speeds to double carriage roads with a speed limit of 80Km/s to normal single lane roads with a speed limit of 60Km/h right through to badly surfaced roads, full of potholes and requiring a 4x4 with a max top speed of 40Km/h - Sandton, for example.
These roads all can have varying traffic conditions on them. Importantly, road builders don't make cars.
3. Traffic Conditions - Clearly these can vary from completely open to fully congested.
If you have a Porsche 911 on the autobahn, with no traffic - you can probably get to it's 300Km/h top speed. If it's Monday morning, you'll be lucky to get to 80Km/s. Neither car nor the road changed, only the level of congestion. If you have the same car on an open rural road, you'll probably also only get 80Km/h. Again the car did not change, the road surface did. And so forth.
In networking we can have the same 3 parameters apply:
1. Modems - think of cars. Different models have different top speeds. But any modem (car) can work on any network (road). Clearly within reason.
2. Networks - think of road systems. You get a 14.4, 21.6, 43.2 network. Just like you get 40, 60, 80, 120Km/h roads. Yet any modem can 'drive' on any of these networks.
3. Congestion - same concept.
Now you can make the analogy of the 'fastest' modem (car) driving on the 'fastest' network (road system) with the least amount of congestion. This is your best bet to ever max out your modem (car). Take that same 43.2 modem onto a slower 14.4 network and your top speed has to drop. Add in some congestion and again top speed will drop.
So, the analogy holds quite well.
Question is why do few people see this as a valid traffic model but not a valid networking model? The only answer I could come up with is that you tend to buy the car from a different supplier than the road system contractor. I.e. if you bought your car from the same people who made the road, would you expect them to give you a road that can use the car's top speed all the time? I actually think, as consumers, we would.
