From the profile, gravel and road bikes aren’t all that different (gravel on the left, road on the right):
If you really know what you’re looking at, though, there are plenty of differences – especially considering both bikes in the photos are the same size. Look at the slope of the top tube, a lot steeper on the gravel bike. This allows for a more relaxed posture in the cockpit (the area between the saddle and the tip of the brake levers). Look at the height of the hoods in relation to those of the road bike. Disc brakes on the gravel, rim on the road. You don’t want to use caliper or cantilever rim brakes on dirt roads, the mud will mess up your rim. Disc brakes save the wheels. The gravel bike also has a shallow drop bar. I don’t like that feature, but whatever…
Those are just the simple differences, though. Let’s look a little deeper at this. Another huge difference is the clearance for the tires. If you look at my Trek, the widest tire that the frame will accommodate is a 24 mm (I tried 26 and it rubbed the frame). My gravel bike has 28’s and could go up to 32:
Also note the tread on the gravel bike (top) contrasted with the road tire (bottom). That little bit of tread provides better traction on dirt roads.
There’s more. Look at the clearance I’ve got on the aforementioned Trek with 24 mm tires on it:
I simply can’t get more tire in there without having to worry about rubbing the frame. There’s a mountain of clearance on my gravel bike – and mine is a 2017, the 2018 Specialized Diverge gravel bikes offer a clearance that can take up to a 42 mm tire. That’s a lot of tire, baby.
Some of the greatest differences don’t show up in a photo, unless it’s a close-up. The gravel bike is a stout 24 pound, aluminum bike while the road bike is carbon fiber and tips the scale at a scant 17 pounds – including pedals, cages and computer. While the carbon fiber frame soaks up a lot of the road vibration, the tires on the gravel bike have to take that responsibility because the frame is as stiff as you get. With skinnier, high-pressure tires, you’d feel every piece of dirt on a paved road. With the 28’s at 55 psi, the gravel bike rides like a dream. Pump those up to the max allowable pressure of 85 psi and the ride would be ugly. I tried it. I didn’t like it – especially when hitting an unexpected pothole.
So, those are some of the bigger differences between a road and a gravel bike. I didn’t bother with the minutiae, such as longer head tubes because I didn’t want the length of this post to get out of hand. Here’s what’s important: Gavel bikes are a load of fun and if traffic scares you, there’s no better place better to ride, with the exception of a trail or a track/velodrome. The only downside is that they’re slow. First, the road surface won’t be conducive to speed. Add to that, width of tire, weight of bike, and decreased aerodynamics knock a couple of mph off of my average.