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I'm currently on a road trip to LA to see a show.

Trying to find parking in downtown LA is quite the challenge. And it's made much worse when I saw a sign saying "Warning: Do not back up - severe tire damage" as I attempted to worm my way out of a full parking lot. I looked down and saw a row of large tire spikes.

These spikes are installed at entrances and exits of all the public parking lots I've visited in downtown LA. Usually there's one at the entrance and another at the exit. They retract down into the ground, and are curved in a way so that cars driving in the "correct" direction would push them down harmlessly while a car diving in the "wrong" direction would have its tires impaled on the spikes.

Now I'm from Philadelphia. I've never seen such blatantly dangerous and harmful access control devices in downtown Philadelphia. Here it's either a powered lift/tilting gate or a speedbump-like device. The other type of parking lot access control device I've seen is a row of powered bollards I saw in London and Cambridge (Great Britain).

Why do many (if not all) parking lots in downtown LA use retractable tire spikes as access control devices? What would be the benefit of a impaling a car on the spikes, causing damage to pretty much all parties involved (the driver/car owner, the parking lot operator, and the people trying to enter/exit the lot)?

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    I don't have any hard evidence for this, but I suspect part of the reason they are more popular in LA than other places is climate; these things could easily be a hidden hazard in the snow in Philadelphia. LA is also home to large numbers of smallish parking lots, where it's common to want to enforce traffic control rules (and payment) without lots of staff or expensive powered gate arms. – Zach Lipton Dec 5 '17 at 5:26
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    Surely all that such spikes do is stop people using entrances and exits in the wrong direction? They don't actually ever interdict passage completely? – AakashM Dec 5 '17 at 9:14
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    I have seen them in the UK on the exit so people can still leave via the exit but not enter while the true entrance is closed at night. – mdewey Dec 5 '17 at 9:51
  • That's strange, user7 - those things were invented in the US and I've seen them everywhere in the US. Maybe there's a special law against them or some such in your state?? – Fattie Dec 5 '17 at 16:41
  • At least one rental car operator at PHL airport uses spikes. Also, take a look at the gates at the US Mint on 4th Street some time. It's not categorically not in our fair city, just uncommon. – user662852 Dec 6 '17 at 19:46
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There are several reasons:

  • Free flow of one-way traffic. Gates will cause backups when opening, especially in confined entryways.
  • Minimal vehicle damage. Other then replaceable tires, spikes will not damage most vehicles unless they are left in Track mode.
  • They are 'multi-use' :). If vehicle crashes a gate, the gate will probably be damage and require repair, taking it out of service for some time. Spikes are not damaged when used and remain in service.
  • Spikes are more effective theft prevention. A gate does very little to stop a vehicle while spikes render it mostly undrivable.
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These are probably design decisions that we don't have access to, but I can see a couple of advantages to the tire spike system:

  • It almost certainly is less expensive to install, maintain, and operate than a powered system like a gate or bollards.
  • The potential to damage a car may be seen as a feature, as it may discourage people from trying to circumvent the system.

Also, as noted by Zach Lipton in a comment, tire spikes may be problematic in regions that get snow (though I have seen them in the northeastern U.S. at least occasionally). This may also help explain why they are more common in LA than the other areas you mention.

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