As I headed up 38 toward Hagerstown one sunny Tuesday, I passed a couple of lonely country billboards and (of course) starting thinking about advertising.

More specifically, about where different forms of advertising reach people.

Billboards reach people most often in their cars, radio is in their cars or maybe at work, TV ads largely reach them in their homes, the paper is often read at the kitchen table or in a comfy chair, and the phone book — well, I couldn’t think of anyone who still uses a phone book.

But what about online advertising? Of course it’s on their computer screens at home or at work, but it’s also on their mobile devices — phone, tablet, etc. Where does the Web reach them?

As I continued down the highway I came upon a group of houses, and I thought of their media-hungry inhabitants. I noticed a fair number of satellite dishes, so I knew they liked TV. But what are they watching? And do they have a favorite radio station? What websites are in their bookmark folders? What are they thinking about?

And then it occurred to me: It’s not about where the Web reaches people, but how.

As TV viewers, we’re after particular content. We want to watch a show or the game, so we settle in for a specific time frame and look for our favored programming between the commercial breaks. When we pick up the newspaper, we flip the pages for local news and sports, absorbing the ads strategically placed throughout. Radio offers a stream of songs that we hopefully like, punctuated by salesmen at fairly regular intervals.

When each of us interfaces with the Web we are usually one of two different types of user: The Seeker or The Surfer. Sometimes we are looking for some specific piece or type of information, and sometimes we just wander around seeing what’s to be seen.

But both of these are substantially different from other media: We have to tell the machine what we’re looking for. While traditional media just serves up what it thinks we want, online content and usage is largely driven by what we think we want.

According to Pew Internet Research, more than 80 percent of Americans use the Internet, and more than 90 percent of those use a search engine to find things.

A search engine is useless if you don’t type something into it. If we’re considering a new car we might type in things like “new car reliability reports” or “2018 auto sound systems.” Google (or your favorite alternative) immediately feeds us info based on what it thinks we’re looking for. It’s a relationship we’ve grown comfortable with, and it profits both sides.

And beyond the search engine, consider how we use various websites. We see something we like and we click it, essentially logging us as interested.

Many sites also solicit our actual opinion about various things — and we often tell them. Some sites have us create collections of things that interest us. Social sites take it even further when we choose to post about things that are thinking about.

So where is online advertising reaching us? Wherever we happen to be using the Internet, certainly, but I think another argument could be made.

If we’re telling the Web what we want just by using it, then it’s likely that it knows what’s on our minds.

If we can then market our businesses inside that very process, aren’t we reaching into the mind of the Internet user in a sense? Advertising online is all about targeting, tracking, and ROI, and done right, it’s a marketing vehicle that can speak to very thoughts of people, even as they’re having them.