How to get all intersections between two simple polygons in O(n+k?

Blue Road Reflectors

• What are blue road reflectors for? California has lots of little bumpy reflectors (hemispheres with a radius of about an inch) built into its roads. There are lines of yellow nubbies down the center of really big streets (possibly laid on top of the traditional yellow paint lines) and lines of white nubbies down the sides of freeways (also possibly accompanied by paint). These nubbies are there so that when you start driving off the road or into oncoming traffic, there is an audible and tactile sensation that you should wake up from your drunken stupor and get back into your lane. So those reflectors make sense. They are colored the same way their corresponding lines are painted. My question is about the blue ones. On the small residential street where I live, there are exactly two blue nubby reflectors - but they're square ones. Each is in the middle of an intersection, although there are other intersections on this street that do not have them. The two intersections share no obvious feature - one is four-way, one is three-way; one has a stop sign, one does not; one has a street light near it; one does not. I've seen the reflectors on an assortment of streets, not just in my city. I want to know why these reflectors are blue and what they are supposed to do.

• Answer:

  Hello Lexi, I’m going to go out on a limb and assume that the blue reflectors that you are describing are near a fire hydrant or some other water supply. According to the California Department of Transportation: http://www.dot.ca.gov/hq/traffops/signtech/signdel/chp6/chap6.htm “6-03.4 Location Markers - Fire Hydrants Blue raised reflective pavement markers, although not an official traffic control device, may be placed on a highway, street, or road, to mark fire hydrant and/or water supply locations. They shall not be used for any other purpose.” In an interesting piece of trivia, these markers are also referred to as “Botts’ Dots” in honor of their inventor Elbert D. Botts. You can read more about this at the following web pages: California Department of Transportation FACT SHEET, Botts' Dots http://www.dot.ca.gov/hq/paffairs/about/botts.htm Snopes.com, Botts' Dots http://www.snopes.com/business/origins/bottsdot.htm Snopes also mentions the blue markers: “Botts' Dots come in two types, round and square, and in several colors. Most are white, center markers are amber, wrong-way markers are red, and fire hydrant markers are blue. On most multi-lane freeways, Caltrans uses four white round non-reflective dots in a row, interspersed every 48 feet with a reflective square, along the painted stripes dividing lanes.” Thanks for the interesting question. I also live in California and I never knew the significance of the different colors. Please let me know if you have any questions. Googlenut Search Strategy: Searched California Department of Transportation website for: blue reflectors street reflective street blue Google Search Terms: california blue reflective markers hydrant ://www.google.com/search?q=california+blue+reflective+markers+hydrant&btnG=Google+Search&hl=en&lr=&ie=ISO-8859-1&safe=off