On Saturday, I walked into a Comcast customer service center in order to return a Comcast-owned cable modem. As the picture above shows, the scene was reminiscent of a post office — long lines of disgruntled customers waiting to speak to representatives through bulletproof glass. I wound up spending more than 30 minutes waiting in line — and customers who were doing more than returning equipment had to wait for even longer.

This kind of scene is rare for companies that operate in competitive markets. If customers have to wait in a long line to get service, they'll switch to another store. But many of us don't have much choice. Comcast is the only company that offers high-speed internet access at my current address. So I'm stuck dealing with it despite its poor customer service.

The problem goes beyond long wait times at the customer service center. Back in June, I purchased a cable modem to save the cost of Comcast's rental fee. A Comcast rep promised to send me a box to ship the old one back to the company, but two months later, the box hadn't arrived. So I figured I'd just drop it off in person.

Unfortunately, Comcast has only a handful of locations to serve the entire Washington DC metropolitan area, and there appears to only be one in the District of Columbia. It's on Michigan Avenue in Northeast DC. And when I got there around 2 p.m., it was packed with frustrated customers.

A check of Yelp shows that I'm far from the only one frustrated by Comcast's poor service. Out of 236 reviews on the page for Comcast's Michigan Ave. location, 206 gave Comcast 1 star. While most of the reviews were complaining about installation woes or billing mistakes, a number also complained about the long lines at the center. "Just tell yourself you're going to the DMV, and you'll be pleasantly surprised," wrote one Yelp user last month.

Customers have been complaining about the long wait times for at least the last year. "Terribly understaffed," said a reviewer in September 2013. "Despite being there on a Friday, there were more windows closed than open, leading to long lines." There's even a picture Yelp user "Marco S." uploaded at the same location in 2010:

"Note that once customers actually get inside the building, the line wraps back and forth 3 times," Marco S. wrote. "HIRE MORE PEOPLE!"

I called Comcast to ask about the problem of long wait times. Here's what a spokeswoman told me:

Your recent visit to our Washington D.C. service center is certainly not the experience we want anyone to have. We've been working on a multi-year project to revamp the hundreds of service centers we have around the country to better serve customers. As part of that project, we will be remodeling the Michigan Avenue location and will open another service center in the District in early 2015. We're also introducing more options for customers to manage their accounts, including a new program we're starting to roll out with The UPS Store to make them an authorized Comcast equipment return location.

It's great that the company is working to address the problem. But the question is why has it taken so long? Customers have been complaining about the wait times at this facility for a year or more.

Why monopolies let customer service problems fester

If Comcast were truly committed to improving customer service, there are some obvious things it could do right away. Right now, the Michigan Ave. facility closes at 3 p.m. on Saturday and is closed Sunday. Comcast could expand the facility's hours. It could also hire more workers — despite the overflow crowd of customers, some of the service windows appeared to be unmanned.

As Comcast's Yelp reviews demonstrate, long waits at customer service centers is just one facet of the company's poor service. There are dozens of reviews complaining about technicians who fail to show up during their scheduled 4-hour window, or who don't know how to do their jobs once they do show up. Customers complain about billing mistakes and being sent to collections for equipment they already returned.

One of the great things about competition is it gives companies a sense of urgency about customer service. Bad customer service causes customers to switch to a competitor, which shows up in the company's bottom line. That causes the CEO to demand changes and leads to pressure throughout the company to improve.

I'm sure the people who run Comcast aren't trying to treat their customers badly. But thousands of unhappy customers doesn't constitute an emergency for Comcast the way it would for a company where customers can get better service from another company. So problems have a way of festering for years.

Disclosure: Comcast Ventures is an investor in Vox Media, the parent company of Vox.com.