I must admit. People leaving wide gaps in city traffic drives me nuts. Why? Turn-lanes. People will leave huge gaps and slow well in advance of stop-lights, causing people behind them who want to turn into the left or right turn-lanes to miss the light. The worst are the people at a light, who, with cars to the left and right decide it's prudent to leave five car lengths between themselves and the car ahead of them at the expense of everyone behind who just wants to get into the turn lane so they can make the light.

The gaps really should be speed dependent. So when you're going 20mph, you will have a smaller gap than if you're going 60.

And that applies to stopping, where you should (IMO) have very little gap. For the exact reasons you mentioned.

The key is to be happy that you're helping other people change lanes more easily (often a hard thing to do in traffic). It's not going to make much difference in your arrival time.

I can't remember the last time I was happy sitting in traffic, or feeling empathetic. If I have to slam on my brakes because others are feeling particularly opportunistic, it will only lead to cynicism.

When I maintain gaps, it's not empathy; at best it is a smug feeling of maintaining a better risk management profile than the person deciding to cut.

I long ago gave up viewing driving as an adversarial activity. Slamming on the brakes means I'm not maintaining a safe distance or speed. Any pent up frustration or anger I harbor is reserved for the sap that caused the accident up ahead that slowed everyone down in the first place.

For me, it's not empathy. It's gas mileage. I burn a whole lot less gas when I'm coasting in 2nd gear than when I have to come to a dead stop, accelerate, step on the brakes again, etc.

I've found that usually people who will cut in front of a gap accelerate pretty quick - after all, they're hopping lanes so they can do that. I rarely need to slow down for them, although occasionally some asshole will misjudge distance & speed.

Don't forget a reasonable gap can significantly increase your fuel economy and how long your breaks last. Remember every time you break that's costing you money.

I have a manual transmission, so it's also clutch wear. Plus I get tired of shifting.

And for me, not making myself motion sick.