Why did London's taxicabs have high roofs?

Why did London's taxicabs have high roofs?

If you ever been to London, or if you've seen a movie that was set there, you might remember the iconic black taxicabs that are ubiquitous in that city; the high roofs set them apart from other vehicles on the streets.

The headroom offered did once serve a purpose. When the cars were originally designed, top hats were still de rigueur for a properly dressed gentleman, and the high roofs allowed a man so attired to enter and leave the vehicle without knocking off his hat.