# Chapter 6: The Four Essential Rules After measuring more than a hundred great places, analyzing thousands of dimensions, and discovering the universal patterns that make urbanism work, we arrive at something surprisingly simple: four rules. Four rules that capture perhaps 90% of what makes the difference between places that can thrive for centuries and places that struggle from day one. These aren't arbitrary impositions or theoretical ideals. They're distillations of what already works, refined to essentials. These four rules provide the basic instructions for building adaptable, walkable, lasting places while allowing infinite variety in their expression. Before diving into specifics, let's be clear about what these rules are and aren't. They're not rigid commandments that guarantee success if followed blindly. Cities are too complex for that kind of mechanical certainty. Instead, think of them as principles, like the rules of grammar, which enable clear communication while allowing for poetry. They're also not meant to be followed 100% of the time. Even the best neighborhoods have exceptions. Charleston's South of Broad district, one of America's most beloved neighborhoods, has blocks that violate Rule #1. Commonwealth Avenue in Boston's Back Bay, one of the most beautiful streets in America, falls well outside the suggested range in Rule #3. The goal isn't perfect adherence but general predominance: follow the rules most of the time and you'll create a framework for good urbanism. For example, if 18 of 20 blocks in a neighborhood are compliant, great; but if only 3 of 20 are, that's probably too few. Also, a 605-foot block does not "break" a 600-foot rule. Most importantly, these rules address only the permanent framework, the constitutional order of streets and blocks. They don't dictate architecture, uses, or activities. They create the stage, not the play. ## Rule #1: Block Size > [!Tip] **Blocks should have sides greater than 200 feet and less than 600 feet, with a perimeter less than 1,800 feet.** Of all the rules, this one matters most. Get block size right and you're 80% of the way to a foundation for good urbanism. Get it wrong and almost nothing else can save you. When we compare the median-sized blocks of various neighborhoods, something striking emerges. At first glance, some blocks are a little more angular than others, some have alleys while some don't, but overall they are nearly the same size and shape. In some cases they are indistinguishable: the Garden District of New Orleans and downtown Madison, Wisconsin, utilize the same block. Charleston and Vienna are barely distinguishable from one another. A worldwide pattern is already evident. ![[block-compare-figure-ground.png]] As we discovered in Chapter 5, when neighborhoods from around the world are plotted by their block dimensions, they cluster tightly together, not scattered randomly across the graph but huddled in one concentrated area. This consensus, the product of millions of decisions over centuries, gives us our first and most important rule. The power of proper block size lies in its relationship to the things we actually build. The vast majority of human activities (living, working, shopping, learning) fit comfortably on lots generally 100 to 200 feet deep. A house with a yard needs about 100 feet. An office building might use 150. When blocks are 200-400 feet deep (the most common range in great places), they accommodate two tiers of these human-scaled lots placed back-to-back. Every lot faces a street, maximizing access and visibility. A 300-foot-wide block splits perfectly into two 150-foot-deep lots, or two 120-foot lots with a 20-foot alley and a row of parking on either side of it, or various other useful configurations. If we break it down between each side of a block, the acceptable ranges get even tighter. The average short side range is only 200 feet to 400 feet; the average long side range is larger: 300 feet to 700 feet. The reason for this is that the short side tends to be a function of lot depth. Because blocks are typically composed of two lot tiers and because lot depths tend to fall in the 100 to 200 foot range, this narrows the overall range of block width. Block length, however, is a function of lot width and can change simply by appending more lots side by side. In this case, the block length can easily expand without sacrificing lot efficiency. This is why block length falls within a higher range. But when blocks grow beyond 600 feet, problems begin to cascade. Salt Lake City's 660-foot blocks create lots that are 330 feet deep, far more than most uses require. Property owners inevitably sell off the backs of their lots, creating an inner ring of development that doesn't face any street. These interior properties, lacking street frontage, struggle to attract investment or contribute to city life. ![[salt-lake-large-block.png]] The perimeter limit of 1,800 feet prevents another kind of error. A block can have acceptable sides and still be too big: a 300-by-600-foot block has a perimeter of 1,800 feet, just within limits, but a 600-by-600-foot block has a perimeter of 2,400 feet. That's Salt Lake City territory. The perimeter rule ensures that if one or two sides reach toward 600 feet, the others must be shorter, and it catches superblocks by stealth: blocks that technically meet the side-length requirements but destroy walkability and permeability through sheer size. It is a common misconception to assume that small blocks imply high population density and big blocks imply low population density. As Chapter 5 showed, block size has little relationship to population: from Paragonah, Utah, with 488 people, to Paris, with 2.2 million, block sizes fall within a tight range. That directly refutes one of the most common objections to proper block sizes, that they're only suitable for dense cities. Human-scaled blocks work everywhere, from tiny towns to global capitals. All these cities represent the spectrum of everything ever built on the planet, and yet they're all operating within the same relative block size. This universality extends down to the individual lot. A standard 60-foot by 120-foot lot is a surprisingly versatile unit of land. That lot could accommodate everything from a house to a parking lot to a skyscraper. While the heights and land use intensities vary, the point is that all these uses occupy the same footprint. In plan view, a house with a yard can take up just as much space as a 30-story skyscraper. ![[lot-uses.png]] Here's what makes block size matter so much: its effects multiply throughout the city. Blocks that are too large don't create one problem; they create problems at every scale: - **For individual properties**: Lots become too deep, wasting land or creating awkward subdivisions that don't face streets. - **For neighborhoods**: Fewer blocks mean fewer streets, reducing both connectivity and the amount of "neighborhood" available per unit area. - **For cities**: The pattern becomes locked in by property lines, nearly impossible to fix once built. Alpharetta, Georgia (Chapter 3), shows the result: when blocks are measured in thousands of feet rather than hundreds, walking becomes impossible, every trip requires a car, traffic concentrates on a few arterial roads, and the pattern is locked in by development. Yet within the 200-600 foot range, considerable flexibility exists. A 280-by-520-foot block, a common unit in our study, can accommodate dramatically different uses: - In Chicago: Condo towers, townhouses, and bungalows - In Amsterdam: Canal houses and palaces - In Buenos Aires: Apartments and markets - In suburban America: A grocery store with parking - Anywhere: A school, a park, offices, or 16 single-family houses Same dimensions, dramatically different uses, all working successfully. ## Rule #2: Block Geometry > [!Tip] **Blocks should be composed of straight line segments with preference given to rectangles. A long side to short side ratio of 1.5:1 or greater is also preferred.** This might seem mundane, even obvious. Of course blocks should be rectangular—what else would they be? But drive through any suburb built in the last fifty years and you'll see: curved, irregular, amoeba-shaped blocks. The abandonment of rectilinear geometry is one of the clearest markers distinguishing traditional urbanism from its modern substitute. Rectangles dominate human construction for obvious reasons. Look around you right now. The text for this book is organized in rectangular columns. The screen you might be reading it on is rectangular. The room you're in probably has rectangular walls, rectangular doors, rectangular windows. Your desk, your bed, your refrigerator, the ice cubes in the freezer—rectangles are everywhere. This isn't cultural bias or lack of imagination. It's functional optimization refined over millennia. As the Manhattan commissioners noted in 1811, _"straight-sided and right-angled houses are the most cheap to build and the most convenient to live in."_[^1] This was true in ancient Mesopotamia, true in 1811, and true today. When blocks are rectangular, this efficiency propagates throughout the urban fabric. Rectangular lots fit efficiently within rectangular blocks. Rectangular buildings fit efficiently on rectangular lots. Rectangular rooms fit efficiently within rectangular buildings. Each scale nests within the next like Russian dolls, minimizing waste and maximizing utility. But surely curves are more natural, more organic, more interesting? Let's test that assumption empirically. Take four blocks, each with the same area (three acres) but different geometries: a rectangle, an irregular angular block, a triangle, and an oval. Holding the area constant isolates geometry as the only variable. We then tested each shape against three land uses, keeping the physical requirements of each use the same across all four blocks: houses, commercial buildings, and parking. Start with houses. Try to subdivide each block into standard 60-by-120-foot lots: - Rectangle: 16 lots fit comfortably - Irregular: 15 lots, with awkward leftover spaces - Triangle: 14 lots, with unusable sharp corners - Oval: 12 lots, with wasted curved edges ![[geometry-yield-table.jpg]] The geometric efficiency loss is 25% between the rectangle and the oval. Over a 160-acre development, that's 40 acres of underutilized land, worth millions of dollars and representing hundreds of homes that can't be built. The same decline holds for commercial buildings and for parking, as the graph below shows. The further a block departs from the rectangle, the less efficient it becomes. The reason is purely geometrical. Nearly everything we build and live with is rectangular: buildings, beds, books, parking spaces, shipping boxes. These objects pack flat against one another and fit perfectly into corners, leaving no waste of land area. Few buildings in this world are truly curved; most are actually _faceted_, made of rectangular materials (bricks, beams, plywood) turned at each joint along the curve to some tolerance. ![[geometry-yield.png]] But the inefficiency goes beyond simple lot yield. Curved blocks create: - Needlessly complex property boundaries requiring more surveying - Irregular building footprints requiring custom designs - Higher construction costs for curved walls and angled connections - Difficult furniture placement in wedge-shaped rooms - Confused circulation patterns - Higher infrastructure costs for curved utilities - Greater difficulty in creating "street walls" with building facades (the "face" of the city) Remember: these blocks will last for centuries, possibly millennia. Every inefficiency gets multiplied by time. A 25% efficiency loss compounded over 200 years represents enormous waste and inflexibility. Why prefer blocks with a length-to-width ratio of 1.5:1 or greater? The answer emerges from both precedent and practical subdivision. First, simple observation: when we analyzed more than a hundred successful neighborhoods, rectangular blocks dominated, not squares and not irregular shapes. This consistency across cultures and centuries says something about how well rectangles serve human habitation. Second, the challenge of corners. Square blocks create a subdivision puzzle that even Thomas Jefferson couldn't solve elegantly. His 1802 checkerboard plan for Jeffersonville, Indiana, was built on square blocks, and the corner lots became awkward: too small when divided equally, too large when combined.[^2] The geometry fights against efficient subdivision. While some cities like Barcelona make square blocks work through careful design, they require more conscious planning to avoid dead corners and wasted space. Third, and most importantly, rectangular blocks are easier to line with active frontage on all sides. Here's why: imagine a block that's 300 feet wide by 450 feet long. You can cap each short end with a row of lots facing outward, say six 100-foot-deep lots. Then along each long side, you can fill the middle portion with another row of five lots, also facing outward. The result? A block completely lined with front doors, front yards, and active faces. No dead zones, no confused orientations, and no wasted corners. A square block works differently. A 367-by-367-foot square (the same area as our rectangle) is too deep to split into two rows of ordinary lots; each row would be more than 180 feet deep. Line all four sides with 120-foot-deep lots instead, and the corner lots collide while a square roughly 127 feet across is left over in the middle. Many cities put that interior to good use as a municipal alley, a shared parking court, or a garden courtyard, which is what Cerdà intended for Barcelona. But the interior has to be planned, owned, and maintained, and the corners still need special treatment. The elongated rectangle solves this elegantly. The 1.5:1 ratio (or greater) ensures enough length to create a clear "grain" to the block: short ends and long sides that naturally accommodate different lot arrangements. The point is flexibility for whatever future uses emerge, not hierarchy. A 300-by-600-foot block (2:1 ratio) can be subdivided dozens of ways: - Twenty 60-by-150-foot lots for single-family homes - Forty 30-by-150-foot lots for townhouses - Mixed sizes for varied housing types - Large parcels for institutional uses - Any combination as needs change over time Each configuration gives every lot a street front. Each works with standard dimensions of construction. Each can evolve into something else as the city changes. The elongated rectangle offers maximum flexibility within a simple, efficient form. Critics often dismiss rectangular blocks as boring or monotonous. "It's just a grid," they say, as if this were self-evidently bad. Yet the places people pay premium prices to live and love to visit—Manhattan, San Francisco, Back Bay Boston, Georgetown, Savannah, the hundreds of great railroad towns across America—are mostly grids. The function of a street network is to connect people and places efficiently while creating developable land. The rectangular grid does this better than any other geometry. That's why it appears independently in ancient China, classical Greece, Roman colonies, medieval new towns, and American cities. The grid is a case of convergent evolution. The supposed monotony of grids also misunderstands how cities actually develop. The grid provides neutral infrastructure. What gets built on it creates variety. Walk through Manhattan's Upper West Side or Barcelona's Eixample (both rigid grids) and try to feel bored. The regular geometry liberates architecture and activity to create interest. The boring suburbs with their curved streets and irregular blocks? They're boring partly because the geometry tries too hard to be interesting, leaving no room for life to create actual interest. ## Rule #3: Rights-of-Way > [!Tip] **Most rights-of-way should have a width between 40 feet and 80 feet. A limited number of rights-of-way may be larger or smaller.** Streets are not just about traffic. If you think of streets primarily as channels for moving vehicles, you've missed most of what they do. Streets are the public rooms of the city, addresses of buildings, routes of infrastructure, shapers of blocks, and creators of value. Movement is just one function among many. When we measured more than a hundred successful neighborhoods, one width appeared more than any other: 60 feet. From the streets of Paris to those in Charleston, from dense Manhattan to small-town Iowa, 60 feet emerges again and again. Why? Because 60 feet is versatile enough to handle almost any condition: - **Rural**: Two lanes with generous shoulders and drainage swales - **Suburban**: Two lanes, parking on both sides, sidewalks, and tree lawns - **Urban**: Anything from four narrow lanes to two lanes with protected bike lanes and wide sidewalks - **Commercial**: Wide sidewalks for cafes, street trees, on-street parking, and delivery zones The same 60-foot right-of-way that starts as a country road can evolve into an urban thoroughfare without changing its geometry. The boundary lines remain constant while the use within them adapts to changing needs. ![[381st-ave-iowa.png]] ![[bowen-street-providence.png]] ![[41st-street-new-york.png]] While 60 feet is the most common width, the full range of 40 to 80 feet covers everything from a lane to a main street while leaving room to adapt. The rule says "most" rights-of-way should fall within this range, acknowledging that special conditions require special responses. A major, "city-scaled" thoroughfare might need 100 feet or more. A pedestrian alley might work at 20 feet or less. A highway is a different creature entirely, playing by different rules. But these exceptions should be exceptional. When every street is extra-wide "just in case," you don't get a resilient network. You get a wasteland of asphalt that spreads destinations beyond walking distance, costs a fortune to maintain, and deadens city life. The best networks have many small streets, fewer medium ones, and very few large ones. Like the branches of a tree, each scale serves a purpose, but the finest grain does most of the work. The tally below shows the distribution of right-of-way widths across the sample. Each dot is one right-of-way, the bars along the top are the totals at each width, and the gray band marks the standard deviation about the mean, for the sample as a whole in the top row and for each neighborhood below it. Read across the top row and the pattern is plain: the majority fall between 40 and 80 feet, with a heavy cluster around 60. ![[ROW-tally.png]] ## Rule #4: Alleys > [!Tip] **Alleys should be between 10 and 20 feet in width and be present in most, if not all, blocks.** If streets are the public faces of blocks, alleys are their service side: unglamorous but essential. This is the rule most often ignored in modern development. The consequences of this neglect are visible in urban centers and suburbs alike: multiple curb cuts along streets, garbage cans crowding driveways, utility lines marring front yards, garage doors dominating street facades, delivery trucks blocking traffic. Commonwealth Avenue in Boston ranks among America's most beautiful streets: a tree-lined linear park punctuated with monuments and flanked by elegant townhouses. But this beauty depends entirely on the narrow alleys threading between the blocks. These alleys absorb all the necessary ugliness of urban life: dumpsters, utility poles, parking, service access, mechanical equipment. Without them, Commonwealth Avenue would be just another street cluttered with the detritus of daily life. This is the alley's gift: it liberates the street to be beautiful. When you have two faces to every lot (front and back), you can present your best face to the public while handling necessity in private. It's the urban equivalent of a mullet: clean in front, messy in back. ![[alley-diagram-2.png]] ![[alley-diagram-1.png]] Alleys also double the access point to every lot: **For residential properties**: The garage goes in back, freeing the front for porches, gardens, and social life. Trash collection happens out of sight. Utilities enter from the rear, eliminating front-yard power lines. **For commercial properties**: Deliveries occur in back, keeping sidewalks clear for customers. Dumpsters hide behind buildings instead of occupying frontage. **For the street network**: Every alley adds to total connectivity, providing alternative routes for local trips. Service vehicles use alleys instead of blocking travel lanes. The finer grain of circulation reduces congestion on main streets. Without alleys, every lot needs its own curb cut, a driveway piercing the sidewalk. On a typical block face of 400 feet with 50-foot lots, that's eight driveways, and sixteen on the block's two long faces: sixteen conflict points between cars and pedestrians, sixteen breaks in the comfortable walking rhythm. With an alley, those sixteen driveways consolidate to two alley mouths, one at each end of the block. The sidewalk runs uninterrupted. Street trees grow without interference. Parallel parking remains continuous. The pedestrian experience transforms from obstacle course to pleasant stroll. Why 10 to 20 feet? Because precedent said so and because alleys need to accommodate specific functions: **10-12 feet**: The minimum for one-way traffic with occasional passing. Garbage trucks can navigate. Utilities fit. This tight dimension ensures alleys remain secondary, not attractive cut-throughs for speeding traffic. **15-18 feet**: The comfort zone. Two cars can pass. Delivery trucks maneuver easily. There's room for utility poles without blocking access. Some cities permit accessory dwelling units (ADUs) along alleys this wide. **20 feet**: The maximum before alleys start becoming streets. Beyond this, the expectation of sidewalks arises, speeds increase, and the alley loses its service character. No city demonstrates the cost of missing alleys more clearly than Manhattan. When the commissioners laid out their famous 1811 grid, they made one catastrophic error: they eliminated alleys to maximize developable land. For over 200 years, New Yorkers have paid for this mistake. Every day is trash day somewhere in Manhattan. Plastic bags pile on sidewalks, creating obstacle courses for pedestrians, leaching smells, attracting rats, destroying the elegance of otherwise beautiful streets. Deliveries block traffic lanes because there's nowhere else to stop. Large buildings need loading docks carved from ground-floor space. The bitter irony is that the commissioners saved perhaps 8% of land by eliminating alleys.[^3] But the loss of efficiency, daily friction, aesthetic degradation, and sheer unpleasantness imposed on millions of people over centuries—the cost defies easy calculation. To fix it now would require assembling slivers of the most expensive real estate on Earth. So the garbage remains, piled on the sidewalks of even the richest neighborhoods, a daily reminder that some planning mistakes are forever. ![[no-alleys-manhattan.png]] ## The Rules Work Together These four rules work together as an integrated system. Proper block sizes create the need for appropriate street widths. Rectangular geometry makes both efficient. Alleys make it all work smoothly. The rules reinforce one another, and Manhattan shows how much one omission can cost even when the other three are right. Together, these rules create a framework for urbanism that has proved successful across cultures and centuries, with room inside it for everything from Savannah to Barcelona. How should planners, developers, and citizens use these rules? Not as rigid commandments but as default settings. Start with the assumption that blocks will be 200-600 feet, mostly rectangular, served by 40-80 foot streets with 10-20 foot alleys. Then, if special conditions require variation, vary consciously and sparingly. This reverses current practice, where every development invents its own geometry. Instead of proving why you should follow time-tested patterns, you must justify why you should deviate. The burden of proof shifts to the innovation, not the tradition. These four rules capture perhaps 90% of what is needed for a successful foundation of urban form. The remaining 10% (the art of street networks, the placement of parks, the response to topography) matters too, and the next chapter takes it up. But without getting these basics right, no amount of artistry can save a place. --- Continue to [[Chapter 7 - Design Principles]] → [^1]: _Remarks of the Commissioners for Laying out Streets and Roads in the City of New York, Under the Act of April 3, 1807_ (1811). [^2]: Jefferson's plan alternated built and open squares in a checkerboard. See John W. Reps, _The Making of Urban America_ (Princeton: Princeton University Press, 1965). [^3]: A 16-foot alley through a 200-foot-deep block takes 8% of its area. --- Annotations: 0,25230 SHA-256 77aa85a1a0cefd2c6f6e &Claude: @Paul Knight: ...