Windblade is in the backseat of the car, but she isn't just in the car, they're stuck in severe traffic. The traffic is so bad that they're only moving 20 mph, but Windblade experiences the urge to pee at the wrong time. She's been having to go since before getting into the car, but she didn't anticipate that she would wound up being stuck in traffic and being delayed, by at least 40 minutes.

For some ponies, this isn't a big deal, but with ponies like Windblade who's bladder acts small and can be overactive, it doesn't take long for the urge to pee to become desperate, and we all know that having the urgent need to pee is uncomfortable and painful. So now, while in the middle of traffic and several miles off from the exit to the destination, Windblade is having a hard time with her almost full bladder (although she's been feeling the urge since before it reached the halfway point) and the urge is unbearable to ignore.

After a little while, Windblade isn't able to mask her problem anymore and asks the driver to take the next exit so that they can stop somewhere so that she could relieve herself, but it seems like the traffic is so bad that it's very hard to move several lanes for the next exit. This is bad news for the Pegasus.

And yes, I'm implying that it's traffic similiar to Los Angeles (I'm been through that traffic during rush hour, it's awful! The traffic can get so bad that it takes three hours to get out of it!). And the background, I took from google.