diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9487718..5270ad5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+## 1.16.8
+
+* Fix some code that could sometimes try to divide by zero
+* Add check for $TIPPECANOE_MAX_THREADS environmental variable
+
 ## 1.16.7
 
 * Fix area of placeholders for degenerate multipolygons
diff --git a/README.md b/README.md
index 5162d2f..38bd37f 100644
--- a/README.md
+++ b/README.md
@@ -147,6 +147,12 @@ resolution is obtained than by using a smaller _maxzoom_ or _detail_.
  * -pt or --no-tiny-polygon-reduction: Don't combine the area of very small polygons into small squares that represent their combined area.
  * -q or --quiet: Work quietly instead of reporting progress
 
+Environment
+-----------
+
+Tippecanoe ordinarily uses as many parallel threads as the operating system claims that CPUs are available.
+You can override this number by setting the `TIPPECANOE_MAX_THREADS` environmental variable.
+
 Example
 -------
 
diff --git a/geometry.cpp b/geometry.cpp
index 27a8b6d..ccf8503 100644
--- a/geometry.cpp
+++ b/geometry.cpp
@@ -13,6 +13,7 @@
 #include <mapbox/geometry/multi_polygon.hpp>
 #include <mapbox/geometry/wagyu/wagyu.hpp>
 #include <mapbox/geometry/wagyu/quick_clip.hpp>
+#include <mapbox/geometry/snap_rounding.hpp>
 #include "geometry.hpp"
 #include "projection.hpp"
 #include "serial.hpp"
@@ -344,29 +345,41 @@ static int pnpoly(drawvec &vert, size_t start, size_t nvert, long long testx, lo
 }
 
 void check_polygon(drawvec &geom, drawvec &before) {
-	for (size_t i = 0; i + 1 < geom.size(); i++) {
-		for (size_t j = i + 1; j + 1 < geom.size(); j++) {
-			if (geom[i + 1].op == VT_LINETO && geom[j + 1].op == VT_LINETO) {
-				double s1_x = geom[i + 1].x - geom[i + 0].x;
-				double s1_y = geom[i + 1].y - geom[i + 0].y;
-				double s2_x = geom[j + 1].x - geom[j + 0].x;
-				double s2_y = geom[j + 1].y - geom[j + 0].y;
+	geom = remove_noop(geom, VT_POLYGON, 0);
 
-				double s, t;
-				s = (-s1_y * (geom[i + 0].x - geom[j + 0].x) + s1_x * (geom[i + 0].y - geom[j + 0].y)) / (-s2_x * s1_y + s1_x * s2_y);
-				t = (s2_x * (geom[i + 0].y - geom[j + 0].y) - s2_y * (geom[i + 0].x - geom[j + 0].x)) / (-s2_x * s1_y + s1_x * s2_y);
-
-				if (t > 0 && t < 1 && s > 0 && s < 1) {
-					printf("Internal error: self-intersecting polygon. %lld,%lld to %lld,%lld intersects %lld,%lld to %lld,%lld\n",
-					       geom[i + 0].x, geom[i + 0].y,
-					       geom[i + 1].x, geom[i + 1].y,
-					       geom[j + 0].x, geom[j + 0].y,
-					       geom[j + 1].x, geom[j + 1].y);
+	mapbox::geometry::multi_polygon<long long> mp;
+	for (size_t i = 0; i < geom.size(); i++) {
+		if (geom[i].op == VT_MOVETO) {
+			size_t j;
+			for (j = i + 1; j < geom.size(); j++) {
+				if (geom[j].op != VT_LINETO) {
+					break;
 				}
 			}
+
+			if (j >= i + 4) {
+				mapbox::geometry::linear_ring<long long> lr;
+
+				for (size_t k = i; k < j; k++) {
+					lr.push_back(mapbox::geometry::point<long long>(geom[k].x, geom[k].y));
+				}
+
+				if (lr.size() >= 3) {
+					mapbox::geometry::polygon<long long> p;
+					p.push_back(lr);
+					mp.push_back(p);
+				}
+			}
+
+			i = j - 1;
 		}
 	}
 
+	mapbox::geometry::multi_polygon<long long> mp2 = mapbox::geometry::snap_round(mp, true, true);
+	if (mp != mp2) {
+		fprintf(stderr, "Internal error: self-intersecting polygon\n");
+	}
+
 	size_t outer_start = -1;
 	size_t outer_len = 0;
 
diff --git a/main.cpp b/main.cpp
index 30cc4a7..a666107 100644
--- a/main.cpp
+++ b/main.cpp
@@ -117,7 +117,14 @@ void checkdisk(struct reader *r, int nreader) {
 };
 
 void init_cpus() {
-	CPUS = sysconf(_SC_NPROCESSORS_ONLN);
+	const char *TIPPECANOE_MAX_THREADS = getenv("TIPPECANOE_MAX_THREADS");
+
+	if (TIPPECANOE_MAX_THREADS != NULL) {
+		CPUS = atoi(TIPPECANOE_MAX_THREADS);
+	} else {
+		CPUS = sysconf(_SC_NPROCESSORS_ONLN);
+	}
+
 	if (CPUS < 1) {
 		CPUS = 1;
 	}
@@ -708,6 +715,9 @@ void radix1(int *geomfds_in, int *indexfds_in, int inputs, int prefix, int split
 					unit = max_unit;
 				}
 				unit = ((unit + page - 1) / page) * page;
+				if (unit < page) {
+					unit = page;
+				}
 
 				size_t nmerges = (indexpos + unit - 1) / unit;
 				struct mergelist merges[nmerges];
diff --git a/man/tippecanoe.1 b/man/tippecanoe.1
index 2c8efba..279681d 100644
--- a/man/tippecanoe.1
+++ b/man/tippecanoe.1
@@ -188,6 +188,10 @@ which may not be what you want.
 .IP \(bu 2
 \-q or \-\-quiet: Work quietly instead of reporting progress
 .RE
+.SH Environment
+.PP
+Tippecanoe ordinarily uses as many parallel threads as the operating system claims that CPUs are available.
+You can override this number by setting the \fB\fCTIPPECANOE_MAX_THREADS\fR environmental variable.
 .SH Example
 .PP
 .RS
diff --git a/mapbox/geometry/snap_rounding.hpp b/mapbox/geometry/snap_rounding.hpp
new file mode 100644
index 0000000..ca1e9e0
--- /dev/null
+++ b/mapbox/geometry/snap_rounding.hpp
@@ -0,0 +1,466 @@
+#include <mapbox/geometry/geometry.hpp>
+#include <math.h>
+#include <map>
+#include <set>
+#include <vector>
+#include <algorithm>
+#include <cmath>
+
+namespace mapbox {
+namespace geometry {
+
+template <typename T>
+void add_vertical(size_t intermediate, size_t which_end, size_t into, std::vector<std::vector<point<T>>> &segments, bool &again, std::vector<size_t> &nexts) {
+	again = true;
+	std::vector<point<T>> dv;
+	dv.push_back(segments[intermediate][which_end]);
+	dv.push_back(segments[into][1]);
+	segments.push_back(dv);
+	segments[into][1] = segments[intermediate][which_end];
+	nexts.push_back(nexts[into]);
+	nexts[into] = nexts.size() - 1;
+}
+
+template <typename T>
+void add_horizontal(size_t intermediate, size_t which_end, size_t into, std::vector<std::vector<point<T>>> &segments, bool &again, std::vector<size_t> &nexts) {
+	again = true;
+
+	T x = segments[intermediate][which_end].x;
+	T y = segments[intermediate][0].y +
+	      (segments[intermediate][which_end].x - segments[intermediate][0].x) *
+		      (segments[intermediate][1].y - segments[intermediate][0].y) /
+		      (segments[intermediate][1].x - segments[intermediate][0].x);
+	point<T> d(x, y);
+
+	std::vector<point<T>> dv;
+	dv.push_back(d);
+	dv.push_back(segments[into][1]);
+	segments.push_back(dv);
+	segments[into][1] = d;
+	nexts.push_back(nexts[into]);
+	nexts[into] = nexts.size() - 1;
+}
+
+template <typename T>
+void warn(std::vector<std::vector<point<T>>> &segments, size_t a, size_t b, bool do_warn) {
+	if (do_warn) {
+		fprintf(stderr, "%lld,%lld to %lld,%lld intersects %lld,%lld to %lld,%lld\n",
+			(long long) segments[a][0].x, (long long) segments[a][0].y,
+			(long long) segments[a][1].x, (long long) segments[a][1].y,
+			(long long) segments[b][0].x, (long long) segments[b][0].y,
+			(long long) segments[b][1].x, (long long) segments[b][1].y);
+	}
+}
+
+template <typename T>
+void check_intersection(std::vector<std::vector<point<T>>> &segments, size_t a, size_t b, bool &again, std::vector<size_t> &nexts, bool do_warn, bool endpoint_ok) {
+	T s10_x = segments[a][1].x - segments[a][0].x;
+	T s10_y = segments[a][1].y - segments[a][0].y;
+	T s32_x = segments[b][1].x - segments[b][0].x;
+	T s32_y = segments[b][1].y - segments[b][0].y;
+
+	// http://stackoverflow.com/questions/563198/how-do-you-detect-where-two-line-segments-intersect
+	T denom = s10_x * s32_y - s32_x * s10_y;
+
+	if (denom == 0) {
+		// They are parallel or collinear. Find out if they are collinear.
+		// http://www.cpsc.ucalgary.ca/~marina/papers/Segment_intersection.ps
+
+		T ccw =
+			segments[a][0].x * segments[a][1].y +
+			segments[a][1].x * segments[b][0].y +
+			segments[b][0].x * segments[a][0].y -
+			segments[a][0].x * segments[b][0].y -
+			segments[a][1].x * segments[a][0].y -
+			segments[b][0].x * segments[a][1].y;
+
+		if (ccw == 0) {
+			if (segments[a][0].x == segments[a][1].x) {
+				// Vertical
+
+				T amin, amax, bmin, bmax;
+				if (segments[a][0].y < segments[a][1].y) {
+					amin = segments[a][0].y;
+					amax = segments[a][1].y;
+				} else {
+					amin = segments[a][1].y;
+					amax = segments[a][0].y;
+				}
+				if (segments[b][0].y < segments[b][1].y) {
+					bmin = segments[b][0].y;
+					bmax = segments[b][1].y;
+				} else {
+					bmin = segments[b][1].y;
+					bmax = segments[b][0].y;
+				}
+
+				// All of these transformations preserve verticality so we can check multiple cases
+				if (segments[b][0].y > amin && segments[b][0].y < amax) {
+					// B0 is in A
+					warn(segments, a, b, do_warn);
+					add_vertical(b, 0, a, segments, again, nexts);
+				}
+				if (segments[b][1].y > amin && segments[b][1].y < amax) {
+					// B1 is in A
+					warn(segments, a, b, do_warn);
+					add_vertical(b, 1, a, segments, again, nexts);
+				}
+				if (segments[a][0].y > bmin && segments[a][0].y < bmax) {
+					// A0 is in B
+					warn(segments, a, b, do_warn);
+					add_vertical(a, 0, b, segments, again, nexts);
+				}
+				if (segments[a][1].y > bmin && segments[a][1].y < bmax) {
+					// A1 is in B
+					warn(segments, a, b, do_warn);
+					add_vertical(a, 1, b, segments, again, nexts);
+				}
+			} else {
+				// Horizontal or diagonal
+
+				T amin, amax, bmin, bmax;
+				if (segments[a][0].x < segments[a][1].x) {
+					amin = segments[a][0].x;
+					amax = segments[a][1].x;
+				} else {
+					amin = segments[a][1].x;
+					amax = segments[a][0].x;
+				}
+				if (segments[b][0].x < segments[b][1].x) {
+					bmin = segments[b][0].x;
+					bmax = segments[b][1].x;
+				} else {
+					bmin = segments[b][1].x;
+					bmax = segments[b][0].x;
+				}
+
+				// Don't check multiples, because rounding may corrupt collinearity
+				if (segments[b][0].x > amin && segments[b][0].x < amax) {
+					// B0 is in A
+					add_horizontal(b, 0, a, segments, again, nexts);
+					warn(segments, a, b, do_warn);
+				} else if (segments[b][1].x > amin && segments[b][1].x < amax) {
+					// B1 is in A
+					add_horizontal(b, 1, a, segments, again, nexts);
+					warn(segments, a, b, do_warn);
+				} else if (segments[a][0].x > bmin && segments[a][0].x < bmax) {
+					// A0 is in B
+					warn(segments, a, b, do_warn);
+					add_horizontal(a, 0, b, segments, again, nexts);
+				} else if (segments[a][1].x > bmin && segments[a][1].x < bmax) {
+					// A1 is in B
+					warn(segments, a, b, do_warn);
+					add_horizontal(a, 1, b, segments, again, nexts);
+				}
+			}
+		}
+	} else {
+		// Neither parallel nor collinear, so may intersect at a single point
+
+		T s02_x = segments[a][0].x - segments[b][0].x;
+		T s02_y = segments[a][0].y - segments[b][0].y;
+
+		double s = (s10_x * s02_y - s10_y * s02_x) / (long double) denom;
+		double t = (s32_x * s02_y - s32_y * s02_x) / (long double) denom;
+
+		if (t >= 0 && t <= 1 && s >= 0 && s <= 1) {
+			T x = (T) round(segments[a][0].x + t * s10_x);
+			T y = (T) round(segments[a][0].y + t * s10_y);
+
+			if ((t > 0 && t < 1 && s > 0 && s < 1) || !endpoint_ok) {
+				if (t >= 0 && t <= 1) {
+					if ((x != segments[a][0].x || y != segments[a][0].y) && (x != segments[a][1].x || y != segments[a][1].y)) {
+						warn(segments, a, b, do_warn);
+						// splitting a
+						std::vector<point<T>> dv;
+						dv.push_back(point<T>(x, y));
+						dv.push_back(segments[a][1]);
+						segments.push_back(dv);
+						segments[a][1] = point<T>(x, y);
+						nexts.push_back(nexts[a]);
+						nexts[a] = nexts.size() - 1;
+						again = true;
+					}
+				}
+
+				if (s >= 0 && s <= 1) {
+					if ((x != segments[b][0].x || y != segments[b][0].y) && (x != segments[b][1].x || y != segments[b][1].y)) {
+						// splitting b
+						warn(segments, a, b, do_warn);
+						std::vector<point<T>> dv;
+						dv.push_back(point<T>(x, y));
+						dv.push_back(segments[b][1]);
+						segments.push_back(dv);
+						segments[b][1] = point<T>(x, y);
+						nexts.push_back(nexts[b]);
+						nexts[b] = nexts.size() - 1;
+						again = true;
+					}
+				}
+			}
+		}
+	}
+}
+
+template <typename T>
+void partition(std::vector<std::vector<point<T>>> &segs, std::vector<size_t> &subset, int direction, std::set<std::pair<size_t, size_t>> &possible) {
+	std::vector<T> points;
+
+	// List of X or Y midpoints of edges, so we can find the median
+
+	if (direction == 0) {
+		for (size_t i = 0; i < subset.size(); i++) {
+			points.push_back((segs[subset[i]][0].x + segs[subset[i]][1].x) / 2);
+		}
+	} else {
+		for (size_t i = 0; i < subset.size(); i++) {
+			points.push_back((segs[subset[i]][0].y + segs[subset[i]][1].y) / 2);
+		}
+	}
+
+	if (points.size() == 0) {
+		return;
+	}
+
+	size_t mid = points.size() / 2;
+	std::nth_element(points.begin(), points.begin() + mid, points.end());
+	T median = points[mid];
+
+	// Partition into sets that are above or below, or to the left or to the right of, the median.
+	// Segments that cross the median appear in both.
+
+	std::vector<size_t> one;
+	std::vector<size_t> two;
+
+	if (direction == 0) {
+		for (size_t i = 0; i < subset.size(); i++) {
+			if (segs[subset[i]][0].x <= median || segs[subset[i]][1].x <= median) {
+				one.push_back(subset[i]);
+			}
+			if (segs[subset[i]][0].x >= median || segs[subset[i]][1].x >= median) {
+				two.push_back(subset[i]);
+			}
+		}
+	} else {
+		for (size_t i = 0; i < subset.size(); i++) {
+			if (segs[subset[i]][0].y <= median || segs[subset[i]][1].y <= median) {
+				one.push_back(subset[i]);
+			}
+			if (segs[subset[i]][0].y >= median || segs[subset[i]][1].y >= median) {
+				two.push_back(subset[i]);
+			}
+		}
+	}
+
+	if (one.size() >= subset.size() || two.size() >= subset.size()) {
+		for (size_t i = 0; i < subset.size(); i++) {
+			for (size_t j = i + 1; j < subset.size(); j++) {
+				possible.insert(std::pair<size_t, size_t>(subset[i], subset[j]));
+			}
+		}
+	} else {
+		// By experiment, stopping at 10 is a little faster than either 5 or 20
+
+		if (one.size() < 10) {
+			for (size_t i = 0; i < one.size(); i++) {
+				for (size_t j = i + 1; j < one.size(); j++) {
+					possible.insert(std::pair<size_t, size_t>(one[i], one[j]));
+				}
+			}
+		} else {
+			partition(segs, one, !direction, possible);
+		}
+
+		if (two.size() < 10) {
+			for (size_t i = 0; i < two.size(); i++) {
+				for (size_t j = i + 1; j < two.size(); j++) {
+					possible.insert(std::pair<size_t, size_t>(two[i], two[j]));
+				}
+			}
+		} else {
+			partition(segs, two, !direction, possible);
+		}
+	}
+}
+
+template <typename T>
+std::vector<std::vector<point<T>>> intersect_segments(std::vector<std::vector<point<T>>> segments, std::vector<size_t> &nexts, bool do_warn, bool endpoint_ok) {
+	bool again = true;
+
+	while (again) {
+		again = false;
+
+		std::set<std::pair<size_t, size_t>> possible;
+
+		std::vector<size_t> subset;
+		for (size_t i = 0; i < segments.size(); i++) {
+			subset.push_back(i);
+		}
+
+		partition(segments, subset, 0, possible);
+
+		for (auto it = possible.begin(); it != possible.end(); ++it) {
+			check_intersection(segments, it->first, it->second, again, nexts, do_warn, endpoint_ok);
+		}
+	}
+
+	return segments;
+}
+
+template <typename T>
+linear_ring<T> remove_collinear(linear_ring<T> ring) {
+	linear_ring<T> out;
+
+	size_t len = ring.size() - 1;  // Exclude duplicated last point
+	for (size_t j = 0; j < len; j++) {
+		long long ccw =
+			ring[(j + len - 1) % len].x * ring[(j + len - 0) % len].y +
+			ring[(j + len - 0) % len].x * ring[(j + len + 1) % len].y +
+			ring[(j + len + 1) % len].x * ring[(j + len - 1) % len].y -
+			ring[(j + len - 1) % len].x * ring[(j + len + 1) % len].y -
+			ring[(j + len - 0) % len].x * ring[(j + len - 1) % len].y -
+			ring[(j + len + 1) % len].x * ring[(j + len - 0) % len].y;
+
+		if (ccw != 0) {
+			out.push_back(ring[j]);
+		}
+
+		if (ring.size() > 0 && ring[0] != ring[ring.size() - 1]) {
+			ring.push_back(ring[0]);
+		}
+	}
+
+	return out;
+}
+
+template <typename T>
+multi_polygon<T> snap_round(multi_polygon<T> geom, bool do_warn, bool endpoint_ok) {
+	std::vector<std::vector<point<T>>> segments;
+	std::vector<size_t> nexts;
+	std::vector<std::vector<size_t>> ring_starts;
+
+	// Crunch out any 0-length segments
+	for (size_t i = 0; i < geom.size(); i++) {
+		for (size_t j = 0; j < geom[i].size(); j++) {
+			for (ssize_t k = geom[i][j].size() - 1; k > 0; k--) {
+				if (geom[i][j][k] == geom[i][j][k - 1]) {
+					geom[i][j].erase(geom[i][j].begin() + k);
+				}
+			}
+		}
+	}
+
+	for (size_t i = 0; i < geom.size(); i++) {
+		ring_starts.push_back(std::vector<size_t>());
+
+		for (size_t j = 0; j < geom[i].size(); j++) {
+			size_t s = geom[i][j].size();
+
+			if (s > 1) {
+				ring_starts[i].push_back(segments.size());
+				size_t first = nexts.size();
+
+				for (size_t k = 0; k + 1 < s; k++) {
+					std::vector<point<T>> dv;
+					dv.push_back(geom[i][j][k]);
+					dv.push_back(geom[i][j][k + 1]);
+
+					segments.push_back(dv);
+					nexts.push_back(nexts.size() + 1);
+				}
+
+				// Fabricate a point if ring was not closed
+				if (geom[i][j][0] != geom[i][j][s - 1]) {
+					std::vector<point<T>> dv;
+					dv.push_back(geom[i][j][s - 1]);
+					dv.push_back(geom[i][j][0]);
+
+					segments.push_back(dv);
+					nexts.push_back(nexts.size() + 1);
+				}
+
+				// Last point of ring points back to first
+				nexts[nexts.size() - 1] = first;
+			}
+		}
+	}
+
+	segments = intersect_segments(segments, nexts, do_warn, endpoint_ok);
+
+	multi_polygon<T> mp;
+	for (size_t i = 0; i < ring_starts.size(); i++) {
+		mp.push_back(polygon<T>());
+
+		for (size_t j = 0; j < ring_starts[i].size(); j++) {
+			mp[i].push_back(linear_ring<T>());
+
+			size_t k = ring_starts[i][j];
+			do {
+				mp[i][j].push_back(segments[k][0]);
+				k = nexts[k];
+			} while (k != ring_starts[i][j]);
+
+			mp[i][j].push_back(segments[ring_starts[i][j]][0]);
+		}
+	}
+
+	return mp;
+}
+
+template <typename T>
+multi_line_string<T> snap_round(multi_line_string<T> geom, bool do_warn, bool endpoint_ok) {
+	std::vector<std::vector<point<T>>> segments;
+	std::vector<size_t> nexts;
+	std::vector<size_t> ring_starts;
+
+	// Crunch out any 0-length segments
+	for (size_t j = 0; j < geom.size(); j++) {
+		for (ssize_t k = geom[j].size() - 1; k > 0; k--) {
+			if (geom[j][k] == geom[j][k - 1]) {
+				geom[j].erase(geom[j].begin() + k);
+			}
+		}
+	}
+
+	for (size_t j = 0; j < geom.size(); j++) {
+		size_t s = geom[j].size();
+
+		if (s > 1) {
+			ring_starts.push_back(segments.size());
+			size_t first = nexts.size();
+
+			for (size_t k = 0; k + 1 < s; k++) {
+				std::vector<point<T>> dv;
+				dv.push_back(geom[j][k]);
+				dv.push_back(geom[j][k + 1]);
+
+				segments.push_back(dv);
+				nexts.push_back(nexts.size() + 1);
+			}
+
+			// Last point of ring points back to first
+			nexts[nexts.size() - 1] = first;
+		}
+	}
+
+	segments = intersect_segments(segments, nexts, do_warn, endpoint_ok);
+
+	multi_line_string<T> mp;
+	for (size_t j = 0; j < ring_starts.size(); j++) {
+		mp.push_back(line_string<T>());
+
+		size_t k = ring_starts[j];
+		size_t last = k;
+		do {
+			mp[j].push_back(segments[k][0]);
+			last = k;
+			k = nexts[k];
+		} while (k != ring_starts[j]);
+
+		mp[j].push_back(segments[last][1]);
+	}
+
+	return mp;
+}
+}
+}
diff --git a/version.hpp b/version.hpp
index cb2e45e..b0aa613 100644
--- a/version.hpp
+++ b/version.hpp
@@ -1 +1 @@
-#define VERSION "tippecanoe v1.16.7\n"
+#define VERSION "tippecanoe v1.16.8\n"