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Hills, light, weather and the places around a street

Hills, darkness, rain and fog, driveways, and who lives nearby can make a street harder or easier to use safely. They are context, not proof of what caused a crash.

This page is a draft. Every fact links to its source, so you can check it yourself.

StreetCase reports have a section called Conditions here. Where we have the data, it shows the shape of the ground, the street lights on the map, the weather in the crash reports, and the places people walk to. If that data is not loaded yet, the section says so. These things may make crashes more or less likely. They never prove what caused any one crash. Read them as clues to ask about, not as answers.

Hills and sight lines

A driver needs to see a person in the road in time to stop. Caltrans, the state highway agency, checks this with two heights: the driver's eye 3 1/2 feet above the road, and an object 6 inches tall on the road. (Caltrans Highway Design Manual, Index 201.3)

How far a driver needs to see depends on speed. Caltrans says about 200 feet at 30 mph, 300 feet at 40 mph, and 430 feet at 50 mph. (Caltrans Highway Design Manual, Table 201.1)

At night in a dip, what matters is how far the headlights reach. Caltrans says that where headlights cannot light far enough ahead in a dip, lighting may be considered. (Caltrans Highway Design Manual, Index 201.5)

What StreetCase does. We sample the ground height every 20 meters along each stretch of road from U.S. Geological Survey lidar, which maps the bare ground about every meter. (USGS 3D Elevation Program) We find the steepest part, the hilltops and the dips. Then we check whether the ground itself could hide a 6-inch object from a driver's eye 3.5 feet up, looking up to 400 meters ahead in both directions.

What it cannot show. The lidar shows the bare ground. It does not show parked cars, fences, hedges, trees or signs, and our check does not look around curves. We use the posted speed in place of the speed a road was designed for. So "the shape of the hill alone does not block the view" is not the same as "drivers can see well here."

Darkness and street lights

Three out of four people walking who were killed in 2024 were hit in the dark. (NHTSA, Traffic Safety Facts 2024: Pedestrians) The Federal Highway Administration lists street lighting as a proven safety fix: up to 42% fewer nighttime injury crashes with people walking at intersections. (FHWA, Lighting)

What StreetCase does. In unincorporated Sacramento County we use the county's street light map. In the City of Sacramento we use the city's map. We count the lights within 30 meters of the road and find the longest stretch with no light within 40 meters. We also show what officers wrote in the crash reports: daylight, dark with street lights, or dark with no street lights.

What it cannot show. The light maps show lights today, not on the night of a crash. They can miss lights owned by someone else, and a light that is on the map can be broken. Officers' codes and the maps often disagree. Across the unincorporated county, many crashes coded "dark, no street lights" have a mapped light within 50 meters. We show both, side by side. We do not yet have light maps for Elk Grove, Citrus Heights, Folsom, Rancho Cordova, Galt or Isleton.

Rain, wet roads, fog and flooding

California crash reports record the weather, the road surface, and anything unusual about the road. The road surface code B means wet. A road condition can be flooded, holes or deep ruts, loose material, or a construction zone. (CCRS raw data template, California Highway Patrol)

What StreetCase does. We count crashes where the officer wrote rain or a wet road and compare that share with the whole county. We also look up the nearest airport weather report within 30 minutes of each crash, from four airports: Sacramento Executive, Sacramento International, McClellan and Mather. (Iowa Environmental Mesonet airport weather archive) If that report shows fog, we count it. The Mather station reports far less rain than the other three, so we never use it for rain.

What it cannot show. The nearest airport can be many kilometers from the crash, and the report shows how far. Fog and rain can be different on the street than at the airport, so a clear airport does not mean the street was clear, and the reverse is true too. A crash on a wet road does not mean the rain caused it.

Driveways and side streets

Every driveway is a place where cars cross the sidewalk and the bike lane. The Federal Highway Administration lists corridor access management as a proven safety fix: fewer driveways along city and suburban main roads means 25% to 31% fewer crashes that kill or injure someone. (FHWA, Corridor access management)

What StreetCase does. We count the driveways, parking-lot entrances and side streets on OpenStreetMap that meet each stretch of road. (OpenStreetMap)

What it cannot show. Most home driveways are not on the map, so our count is a floor. The real number is higher.

Who lives and walks here

Places people walk to. We list grocery stores, corner stores, parks, playgrounds, libraries, clinics, hospitals, community centers and senior homes within half a mile, from OpenStreetMap, plus public schools from the state school list. Map data can miss places.

Households with no car. A household with no car gets around on foot, by bike, by transit or with rides from others. We show how many households in the census tract have no car, from the American Community Survey for 2020 to 2024. (Census Reporter) These are estimates. We always show the margin of error, and we say "too few to tell" when the margin is as big as the number.

Pollution burden. CalEnviroScreen is California's tool for finding communities that carry the most pollution and health burden. We show the census tract's CalEnviroScreen 5.0 percentile. (CalEnviroScreen 5.0 results, OEHHA) It describes the whole tract, not one street. The "traffic" part of CalEnviroScreen measures how much traffic pollution reaches homes, not crash risk.

This matters for money. The state's Active Transportation Program must spend at least 25% of its money on projects that benefit disadvantaged communities, and it scores how much each project helps them. One way a community qualifies is being in the worst 25% of the state on CalEnviroScreen 4.0, an older version than the one we show. (2027 Active Transportation Program Guidelines) Check the official list before you cite a place in a grant letter.

How to use this in a letter

• Ask about a condition, do not blame it. "Crashes here keep happening in the dark, and the county light map shows a long stretch with no light. Can you check the lighting?" is fair. "The darkness caused these crashes" is not something the data can show.
• Pair a condition with the crash record. A dip in the road with no crashes is not a priority. A dip where people walking were hit at night is worth a look.
• Use the region page to find stretches with both high crash numbers and high pollution burden, or many households without a car. Those may be good places to look first for grants that favor disadvantaged communities.

Last checked 2026-11-02.

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