Strategies to Shorten attacks?

I don't suffer from attacks very often but that means that I don't know when to expect them (I guess we never know when to expect them otherwise it wouldn't be an attack). I just mean that I don't have particular triggers that I can identify that might set one in motion. Today I had a pretty severe one when I arrived to campus. I was crossing the street and a driver honked his horn at another driver behind me. I'm sure many people would jump, that isn't unusual, but I had a fight or flight response. I could feel my heartrate increasing at an exponential rate where I felt like it could jump right out of my chest and I had this sudden feeling that I needed to get away RIGHT NOW. I started walking faster (not a good idea when it feels like you can't breathe normally) as if someone were following me. Irrationally, I kept looking around to see if I was being chased and the logical part of my brain told me I needed to focus on my breathing but the panic kept yelling at me to go faster. I made it to my building and climbed the stairs to my floor and ran into my office. I immediately felt better when the door was close but not enough to calm down. I couldn't sit down and I wasn't calm enough to do a mediation exercise (which I have been doing regularly lately to calm my general anxiety). Instead I paced my office for over a half hour focusing on slowing my breath.

I hate when this happens (as rare as they are) and I worry because they have been occuring more regularly. Again, there are no discernable triggers (I don't get scared from car horns normally and I have been on this campus long enough to feel safe).

When you have irrational thoughts or you can't calm your mind to practice mediation, what are some other strategies that can be used instead? I can't devote a half hour to an hour each morning to calming myself down incase someone honks on my way to class. I need another solution.