If You Think Traffic is Too Fast in Your Neighborhood, it Probably Is

If you think a street’s traffic speeds are too fast, it has nothing to do with the speed limit. It’s the street. Traffic speed has been a recurring theme at some of the meetings I’ve been to lately. Even on streets with bike lanes, many people refuse to use the bike lane (or the street), because they believe there are too many vehicles going too fast.

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Fair Oaks Blvd in Sacramento, CA, north of Howe Ave, one of the busiest street segments in Sacramento, CA. The street is straight and wide. It’s nearly a mile between traffic lights. Cars go fast and the frontage roads are there so vehicles can turn out of high speed traffic and more easily access the plazas. The frontage roads are also public rights of way, which means the project has a lot of room for solutions like protected bike lanes.

Fair Oaks Boulevard between Munroe Street and Howe Avenue is one of the busiest streets in the Sacramento area, with a 40mph speed limit and six lanes of fast traffic. Its intersection with Howe Avenue has the highest traffic count in the Sacramento area. Fair Oaks Blvd has destinations like apartments and shopping plazas, and its adjoining streets have residential neighborhoods, schools, parks, and bike lanes, but Fair Oaks west of Munroe Street has no bike lane and, for 3/4 of a mile until it reaches Howe Avenue, it has no pedestrian crossings either. Sacramento County is, fortunately, planning to improve bicycle and pedestrian safety, access, and mobility (you can see the project funding application here) by adding crosswalks with signals, bike lanes (hopefully protected bike lanes), and other features.

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Fair Oaks Blvd north of Howe Ave

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More of Fair Oaks Blvd north of Howe Ave

Simply changing the posted speed limit will not, however, affect vehicle speeds. Fair Oaks Blvd is a fast street, because it’s built fast. It’s straight and goes nearly a mile without traffic signals. It has six lanes of traffic with center turn lanes and frontage streets for traffic to pull off and access the plazas. The majority of drivers on Fair Oaks Blvd drive around 40 mph not because of the speed limit, but because because 40mph is, intuitively, a reasonable speed for Fair Oaks Blvd.

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A view north on Fair Oaks Blvd. It’s a wide street, much of it with three lanes in each direction. The wide street encourages high speeds, has no bike lanes, and is difficult for pedestrians to cross safely.

Statutes dictate specific speed limits for certain types of streets, such as the 15mph speed limit through alleys and the 25mph speed limit near schools and in residential and commercial districts. Local governments have authority to establish speed limits for “local” roads within their jurisdictions, but in California this authority is limited, because the Vehicle Code also prohibits “speed traps” (People v. Goulet (1992) 13 Cal.App.4th Supp. 1.) A “speed trap” is a roadway section whose speed limit is not based on an engineering and traffic survey conducted within five (and sometimes seven) years and where enforcement involves the use of radar or other electronic devices that measure speed (courts have also included visual estimates). (Vehicle Code Section 40802.) In addition to showing that a traffic and engineering survey has been conducted within five years, the prosecution must also establish that the engineering and traffic survey justifies the speed limit.

The Manual of Uniform Traffic Control Devices and the California Manual for Setting Speed Limits also provide specific procedures for setting speed limits. An engineering and traffic survey starts by monitoring the 85th percentile speed of free-flowing traffic, usually at the peak hour on two or three week days. In other words, equal to the fastest 85th percent of traffic (about one standard deviation above the mean). The speed limit is normally then set at the first five mph increment below the 85th percentile. However, in matching existing conditions with the traffic safety needs of the community, engineering judgement may indicate the need for a further reduction of five miles per hour. Factors justifying further reduction, which are described in Vehicle Code Section 627, include –

  • Accident records;
  • Highway, traffic, and roadside conditions not readily apparent to the driver;
  • Residential density (under certain conditions);
  • Pedestrian and bicycle safety

Courts require strong documentation to support a speed limit more than 5 mph below the 85th percentile speed in order to avoid making violators of a disproportionate number of the reasonable majority of drivers. The County in Goulet attempted to justify the speed limit based on “limited sight distance to commercial driveways.” However, the survey lacked any information regarding sight distance to driveways and did not explain how such conditions would affect safe speeds. No speed-related accidents had occurred in at least three years. As a result, the survey lacked sufficient information to support the speed limit and the court ruled the County’s action a speed trap.

For the vast majority of roads, the 85th percentile speed matches with speed limits to 5mph, although some local governments round up, rather than down. While the 85th percentile speed is the primary factor in setting speed limits across the US, surveys have found that on many roads the 85th percentile speed, and often even the mean operating speed, far exceeds the speed limit. The radical increase in operating speeds, particularly in busy urban areas, has led to many traffic engineers (among others) to question whether the 85th percentile speed value is an appropriate tool for setting speed limits. Drivers today are increasingly rushed, cars have more power, better safety systems, and drive more comfortably at higher speeds, speed limits are often viewed as a minimum or “advisory,” and drivers rarely expect traffic citations for exceeding the speed limit by 5-10mph on urban collectors and arterials. Notably, the 5-10mph “cushion” perceived by many drivers matches the 5-10mph range in which an engineering and traffic survey needs much more information in order to support a lower speed limit. Many urban arterial and collector roads are clearly overbuilt with the goal of moving as much traffic as fast as possible and provide a safer design for drivers who choose to driver fast. They have wide streets, wide lanes, and long, clear lines of sight that encourage high speeds. Unfortunately, higher urban design speeds degrade city streets and walkable neighborhoods by mandating larger curb radii, wider travel lanes, guardrails, no on-street parking, and generous clear zones. Speed trap laws exist to prevent local governments from using deceptive speed limits as tools to unfairly generate revenue.

The bigger problem is that, “free-flowing” traffic, by definition is traffic not obstructed by traffic controls, pedestrians, bicycles, or any slow-moving vehicles. As a result, the 85th percentile of that speed does not account for pedestrians, bicycles, and other factors a local government may consider in setting speed limits. As a result, even if local government posts a speed limit 5-10 mph below the 85th percentile, the 85th percentile speed would remain too fast for the conditions and uses of the streets. Just changing the posted speed limit does not change driver behavior, because drivers still view the street as “fast.” A court would enforce the speed limit, but the local government would need to devote substantial all enforcement resources.

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North 12th Street at Richards Blvd southbound in Sacramento is a transition zone for Highway 160 into the downtown area. It has a posted speed limit of 35 mph due to light rail tracks and an uncontrolled crosswalk, but the most recent traffic and engineering survey, conducted in 2011, measured the 85th percentile speed at almost 42 mph, 7 mph above the posted speed limit. As a result, the majority of vehicles routinely exceed the speed limit, even during peak periods.

Local governments seeking to better accommodate bicycling and walking re-engineer their streets. They start with a target speed that should be safe and reasonable for all planned uses and then design the street so that the 85th percentile of drivers will drive that speed. For example, the National Association of City Transportation Officials recommends 35 mph speed limits for most urban arterials and 30 mph for collectors. They reduce traffic lanes (often called a “road diet”), narrow the travel lanes, and use the additional space for bike lanes and potentially street parking. If further necessary, they may also add features like center medians, speed tables, curb extensions, chicanes, speed bumps, and traffic circles. This way, the character of the street and surrounding environment should “cue” the 85th percentile of drivers to drive the target speed. Since the speed limit is then closer to the target speed, the street has a more legally supportable speed limit and, since more people drive the intended speed, traffic engineers can time lights to manage traffic flow more efficiently.

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A Complete Streets concept the County could apply to Fair Oaks Blvd

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Another Complete Streets concept the County can apply to Fair Oaks Blvd.

The County will likely need to reduce vehicles speeds to make Fair Oaks Boulevard a “complete street” that “creates a sense of place for the community and visitors,” encourages multimodal transportation, and functions as a bicycle transportation corridor. Changing the posted speed limit will not, on its own, change driver behavior and could easily run afoul of speed trap laws. For the surrounding land use to change, the street itself needs to change. Fortunately, the wide public right of way created by the frontage roads create opportunities for solutions like protected bike lanes.

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Complete Streets concepts for Fair Oaks Blvd that would accommodate all users , including cars, buses, pedestrians, and people on bikes.

Fair Oaks Boulevard is just one of many streets in the Sacramento area in need of such changes. Fair Oaks south of Howe Avenue will need to change in order to create a continuous bicycle transportation corridor to CSU Sacramento into Downtown. In the Downtown area, the streets, much like Fair Oaks Boulevard, are designed for for moving traffic into, through, and out of Downtown. The King’s arena is a start, but if the City is serious about renewing the Downtown area and making it a destination, it too will need to remake the streets, because simply changing the speed limit won’t be enough.



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