California Safe Speeds Toolkit: Research on Speeds, Speed Limits, and Safety

1. What is Safe Speed?

Speed is a significant concern both locally and statewide. According to Caltrans, nearly 3,800 traffic fatalities and 14,500 serious injuries occur in California each year. Approximately 47% of these fatalities and serious injuries are related to speeding and aggressive driving (Caltrans, 2025).

A safe speed is a travel speed that lowers the likelihood of a crash by allowing more response time and better visibility. It also accommodates human mistakes and provides crucial redundancy by maintaining impact energy on the human body at a tolerable level in case of a crash (FHWA, 2022). The Federal Highway Administration (FHWA) states that Safe Speeds reduce system kinetic energy and accommodate human injury tolerance (FHWA, 2022); an increasingly important goal as vehicles increase in size (Monfort, 2025).

In opposition to the traditional view that drivers can choose reasonable and safe speeds (see Section 2), safe speed limits should instead be set based on “the likely crash types, the resulting impact forces, and the human body’s ability to withstand these forces” (National Transportation Safety Board, 2017). This approach minimizes the likelihood of fatal and serious injuries for all road users, particularly vulnerable road users such as pedestrians and bicyclists (National Transportation Safety Board, 2018; Forbes et al., 2012; Jurewicz et al., 2014). For many jurisdictions across California, this often involves reducing the posted speed limit to a safe speed limit, which is a proven safety countermeasure according to FHWA (FHWA 2022) (see Section 3).

Determining safe speed limits requires considering numerous factors, including potential hazards, the road environment, the presence and movement of different road users, surrounding land uses, and policy goals for traffic safety. Additionally, a safe speed limit cannot be determined in isolation of road design, vehicle design, and the anticipated road users – all elements of a holistic safe system approach. Local jurisdictions can specifically integrate safe speeds and safe road design (see Section 5) to comprehensively improve safety for road users of all modes, ages, and abilities. Several European cities have lowered speed limits to just 30 km/h (~18.6 mph) on many roads, and have seen a 23% reduction in road crashes, a 37% reduction in fatalities, and a 38% in road injuries (Yannis, 2024). Without the ability to employ this more holistic approach, speed limits can sometimes be set higher than is considered safe. For example, prior California speed limit setting procedures pushed Los Angeles to primarily rely on 85th percentile speed data when updating speed limits citywide in 2017, which actually led to speed limit increases on 90 miles of roadways, including several with histories of serious or fatal crashes (Toda, 2018). In contrast, Portland and Seattle both adopted more flexible methodologies that incorporated factors such as crash statistics and road type related to the streets under consideration, and speed limit increases were not an observed outcome in these non-California cities (Toda, 2018).

In response to issues such as those experienced by Los Angeles in 2017, laws and guidelines regarding speed limit setting at the state level have been evolving. Recent state legislation has given jurisdictions additional flexibility to consider factors other than the 85th percentile when setting speed limits (Miller, 2024, p. 49). California Department of Transportation (Caltrans) policy states that different land use, community context, and the associated road user needs should be taken into account when evaluating traffic calming options (Caltrans, n.d.). The Current Speed-Limit-Setting Law section of this toolkit provides information on options for jurisdictions to flexibly set safe speed limits without relying exclusively on the “85th percentile rule.”

2. Limitations of the 85th Percentile Rule for Setting Speed Limits

The “85th percentile rule” was developed in the 1930s and became a widely accepted “conventional wisdom” for setting speed limits across the United States. This approach sets the speed limit based on the actual speed of drivers at the 85th percentile (that is, the speed that 85% of drivers do not exceed); it assumes that the collective judgment of drivers determines what is safe. Proponents have historically argued that it reduces the variability in travel speeds and allows law enforcement to focus on the most extreme cases of speeding (Grembek et al. 2020, p. 27).

An underlying assumption of the “85th percentile rule” is that drivers can and should set safe speed limits. However, recent research, such as Section 4.1 of the UC ITS report by Grembek et al. (2020), increasingly sheds light on the behavioral challenges for drivers (Grembek et al. 2020, p, 46). Streets in urban and some suburban areas present complex challenges for drivers to self-regulate safe speeds; NCHRP 17-76 concluded that the 85th percentile speed is not the safest approach under many urban conditions (Fitzpatrick et al., 2021). Importantly, urban and some suburban streets may need to accommodate not just drivers, but also pedestrians, bicyclists, and other street users. As such, the traditional "85th percentile rule" is not always the best approach for setting speed limits that align with Complete Streets, Vision Zero, Safe System, Active Transportation, Sustainable Transportation, and Transportation Equity policy goals of local, regional, and state jurisdictions.

Behavioral research shows that drivers cannot self-regulate a safe speed easily on lower-speed roads.

Research has shown that drivers can underestimate speed by up to 30% at 35 mph (Grembek et al. 2020, p. 46). These lower-speed roads often lack strong visual cues such as guardrails or shoulder widths for drivers to assess safety and speed (Ben-Bassat & Shinar, 2011). When drivers believe that speeding does not threaten safety, they have a tendency to exceed the speed limit (Mannering, 2009).

Drivers’ underestimation of speed worsens in adverse weather conditions.

Research has shown that the driver’s perception of speed decreases in fog. As a result, drivers think they are driving far more slowly than they actually are in foggy conditions (Snowden et al., 1998).

Many drivers speed and believe that excess speed does not threaten safety.

In a 2023 survey done by the National Highway Traffic Safety Administration, it was determined that 44% of drivers sometimes speed and 17% of drivers almost always speed (Cosby et al., 2023).

About four in ten respondents to a recent survey conducted by the American Automobile Association (AAA) admitted to driving ten miles per hour or more above the speed limit on residential streets, even as 90 percent of them reported being somewhat or completely disapproving of that behavior (AAA Foundation for Traffic Safety, 2026).

Speed enforcement in local areas is limited and patrol-based does not create effective deterrence.

Speed enforcement in local areas is typically more limited than on highways. Accordingly, the perception of the certainty of punishment is reduced and the overall impact of legal sanctions as a deterrence for speeding on local streets is diminished.

However, automated speed enforcement raises the certainty of punishment more reliably than traditional patrol-based enforcement, and early evaluations of two recent speed safety camera pilots, both authorized under AB 645 (2023), support their efficacy. After one year of operation, the San Francisco Municipal Transportation Agency (SFMTA) reported a 79 percent drop in the share of drivers traveling 10 mph or more over the posted limit at camera locations (SFMTA, 2026). After 6 months of operation, Oakland reported a 70 percent reduction in vehicles traveling 11 mph or more over the posted speed limit across its camera locations, with some sites seeing reductions above 90 percent (OakDOT, 2026).

Local area streets are impacted by spatial speed creep from neighboring highways.

Research has shown that higher speeds on some highways can cause higher speeds on connecting local area roads (Casey & Lund, 1992) and also that pedestrian fatality rates are elevated on roads adjacent to interstate highways (Nehiba & Tyndall, 2023). The implication of these studies is that the impact of speed limits on highways can be carried over to local area streets and should be considered as a safety issue.

The safety risks associated with speed are higher for pedestrians and bicyclists.

The National Transportation Safety Board (NTSB) reports that mitigating speed is vital to improving pedestrian safety (NTSB, 2018), and there are unique safety considerations when considering biking as well. Those traveling using active transportation modes such as biking and walking are at great risk of serious injury in the event of a crash with a motor vehicle given vehicle size and speed relative to the bicyclist or pedestrian. Safer speeds can not only reduce the risks to those walking and biking on the streets but also to the operators of motor vehicles who are involved in a crash given the strong relationship between speed and injury severity.

The 85th Percentile Rule may lead to speed creep in specific situations.

Consider a situation where, collectively, drivers elect speeds such that about half of them drive faster than the speed limit. This behavior, if coupled with a periodic application of the 85th percentile rule, may cause an upward drift in speeds (Grembek et al. 2020, p. 44). This sequence of events leads to what is known as “speed creep.” It may be important to systematically distinguish roads that are experiencing speeds that are higher than what is safe from other roads where speed limits are set accurately to better identify which roads are in need of more flexible methodologies for setting speeds. It is also important to note that even a 0.1 mph increase in the observed 85th percentile speed could lead to an increase in a speed limit by 5 mph if the 0.1 mph increment changes how the 85th percentile speed is rounded. Consequently, this could also lead to speed creep.

3. State-of-the-art Evidence on Speed & Safety

The connection between speed and safety is well-established from the perspectives of both physics and human behavior. According to Newtonian physics, vehicles with higher speeds have more kinetic energy to transfer to another person, vehicle, or object. This transfer of energy during a crash is the root cause of traffic injuries and fatalities, and this kinetic energy transfer can cause serious damage when a crash involves a bicyclist or a pedestrian. The higher the impact speed of a crash, the greater the risk of serious injury or death.

As vehicle size and weight continue to grow, managing system kinetic energy has become an increasingly imperative requirement for road safety. Recent research by Monfort and Mueller highlights how modern vehicle fleet designs, particularly taller, blunter front ends, increase the risk of serious pedestrian injuries and fatalities even at moderate speeds. Below, Figure 1 outlines the fatality risk from a crash at varying speeds.

Figure 1. Risk of pedestrian death in relation to impact speed

Infographic detailing 5 scenarios where a driver strikes a pedestrian at different speeds (20, 30, 40, 50 and 60 miles per hour) and the resulting fatality rates (1, 8, 39, 82, 97 percent respectively).

Front-end vehicle height significantly compounds the danger posed by higher travel speeds. When comparing impact speed increases from 15 mph to 35 mph, serious injury risk increases from 9% to 52% for median-height passenger cars, while serious injury risk dramatically spikes from 11% to 91% for median-height pickup trucks and sport utility vehicles (SUVs) (Monfort, 2025).

In terms of human behavior, higher speeds increase the amount of information that drivers must process, leading to more driver stress and fatigue. Higher speeds also narrow drivers’ field of vision and require greater distances for drivers to react and come to a complete stop (Figures 2 and 3). Speed limit changes that slow down cars can reduce the reaction and braking distances needed to safely yield to pedestrians and bicyclists (NHTSA, 2015).

Figure 2. As speed increases, peripheral vision decreases

Infographic detailing 3 scenarios at 20, 30 and 40 miles per hour and how the driver’s field of vision, displayed as a semi circle becomes smaller as speed increases.

Source: https://visionzeronetwork.org/resources/safety-over-speed/

Figure 3: As speed increases, stopping distance increases

Infographic displaying 3 different speed scenarios (20, 30 and 40 miles per hour) and how speed affects braking distance. At 20 miles per hour the braking distance is 63 feet, at 30 it is 119 and at 40 it is 164.

Adapted from Vision Zero Network https://visionzeronetwork.org/resources/safety-over-speed/

Below are summaries of evidence-based findings on the relationship between speed limits and safety outcomes, as well as areas where research is ongoing or unclear:

Speed Limit Reductions Lower Mean Speeds & Reduce Crashes: A 5 mph reduction in speed limit is likely to decrease mean vehicle speed by 1-2 mph, or by 3 mph with stronger enforcement (Elvik et al., 2019). It may also reduce the speed of the fastest drivers to a much greater extent (Silvano & Bang, 2016). The overall effects of a 5 mph speed limit reduction can lead to a 10-30% reduction in all fatalities and 2-15% reduction in serious bicyclist injuries (Elvik, 2009; Helak et al., 2017; Zahabi et al., 2011).
Serious Injury & Fatality Risk Escalates Rapidly: Pedestrian serious injury risk jumps from 18% at 20 mph to 67% at 35 mph, while fatality risk increases from 1% at 20 mph to 19% at 35 mph, with fatality risk exceeding 80% at 50 mph. The threshold for a "safe" impact speed (≤10% serious injury risk) now stands at ~15 mph (Monfort & Mueller, 2024).
Vehicle Size & Front-End Height Compound Danger: As impact speeds increase from 15 mph to 35 mph, serious injury risk rises from 9% to 52% when struck by a median-height passenger car, but dramatically spikes from 11% to 91% when struck by a median-height pickup truck or SUV (Monfort & Mueller, 2024).
Lower Speeds Encourage Active Transportation: Lower vehicle speeds may lead to improved safety perception with more people choosing to walk and bike, which in turn may lead to lower pedestrian and bicyclist crash rates. This phenomenon of “safety in numbers” for pedestrians and bicyclists is a continually evolving field of study (Elvik & Bjørnskau, 2017).

The following diagram shows how the posted speed limit can affect traffic characteristics, which in turn affect safety outcomes. Research outcomes on each of these links are summarized below.

Figure 4. Paths by which speed limits can affect safety outcomes

Flow chart detailing how speed limits affect traffic characteristics and safety outcomes. For more information, please see the following summary.

A (Speed Limit & Traffic Speed): How does the speed limit affect traffic speed?

Research has shown that reducing the speed limit by 5 mph typically results in a decrease in mean speed of 1-2 mph, or by 3 mph with stronger enforcement (equivalent to 60% of the speed limit change) (Grembek et al., 2020; Elvik, 2009; Silvano & Bang, 2016; Islam et al. 2014). This finding is consistent for limited access roadways as well as complete streets with multiple types of road users. Lowering speed limits can also reduce the speed of the fastest drivers (Silvano & Bang, 2016; Grembek et al., 2020).

A (Speed Limit & Traffic Speed) + D (Traffic Speed & Likelihood of Crash): How does the speed limit affect the likelihood of a crash?

Most studies suggest that higher speed limits are correlated with higher numbers of crashes. However, this positive relationship between speed limits and crash incidence is not consistent (Grembek et al., 2020, p. 20), and the variation across study findings makes it difficult to estimate the precise effect of traffic speed on crash frequency. For example, one study found that every 1 mph reduction in average speed can lead to a decrease of 2-7% in crash frequency (Taylor et al., 2000). Another study found that a 1 mph reduction in speed at around 20 mph can lead to a decrease of around 12% in crash frequency (Elvik et al., 2004).

C (Impact Speed & Crash Severity): How does the impact speed affect crash severity?

Research has consistently shown that reducing speeds on roadways is associated with fewer injuries, particularly fatal ones (Grembek et al. 2020, p. 8). Contemporary NHTSA crash data analyzed by Monfort & Mueller (2024) demonstrates how pedestrian fatality and serious injury risks increase rapidly above travel speeds of 20 mph, while highlighting the compounding danger posed by modern vehicle geometry.

At 20 mph, pedestrian fatality risk is 1%, but it rises to 19% at 35 mph and exceeds 80% at 50 mph. Vehicle design significantly amplifies these risks: as impact speeds increase from 15 mph to 35 mph, serious injury risk increases from 9% to 52% for a median-height passenger car, but dramatically spikes from 11% to 91% when struck by a median-height pickup truck or SUV (Monfort & Mueller, 2024).

Figure 5. Pedestrian Injury and Fatality Risk at Varying Speeds

Graph displaying three pedestrian injury risk curves (moderate injury, serious injury, and fatality) by vehicle impact speed in mph. Separate curves illustrate how moderate injury, serious injury, and fatality risk dramatically increases with speed.

Source: Monfort & Mueller (2024 / IIHS)

Pedestrian injury and fatality risk as a function of impact speed (2015–2022 data) (Monfort & Mueller, 2024)

A (Speed Limit & Traffic Speed) + B (Traffic Speed & Impact of Speed) + C (Impact Speed & Crash Severity) and A (Speed Limit & Traffic Speed) + D (Traffic Speed & Likelihood of Crash): How does the speed limit affect the number of injuries and fatalities?

The bulk of research indicates that reducing the posted speed limit by 5 mph, under certain circumstances, can lead to an 8-15% decrease in injuries and a 10-30% decrease in fatalities (Grembek et al., 2020; Elvik et al., 2019), with the strongest effect for middle-range speeds. Some outlier studies have reported reductions in injuries as high as 28% and 39%, stemming from a 5 mph reduction in the posted speed limit (Elvik et al., 2019). Using past empirical evidence, Elvik (2019) concluded that injuries and fatalities relate to speeds exponentially to the fourth power (Elvik et al., 2019). While the exact result of speed limit reduction on injury and fatality frequency can vary with road type, the original posted speed limit, and urban context (Alnawmasi & Mannering, 2022), there is consistent evidence that even modest decreases in average speeds on roadways could have a significant impact on the incidence of serious injury and fatality crashes. One strong example comes from a study of rural roads in Sweden, which found not only that reducing the posted speed limit led to fewer fatalities, but that increasing it led to more fatalities (Vadeby & Forsman, 2018). Another study out of British Columbia, Canada found that increases in speed limits on rural highways lead to an increase in overall crashes and a large increase in the number of fatal crashes (Brubacher et al., 2018).

While it is challenging to isolate the effects of speed limits on safety outcomes, especially for pedestrians and bicyclists, it is abundantly clear that roads with lower speeds pose a lower safety risk for pedestrians and bicyclists. One review of multiple studies found that a 5 mph decrease in vehicle speeds involved in a crash is associated with 56-88% fewer serious pedestrian injuries and 80-96% fewer pedestrian fatalities (Hussain et al., 2019).For bicyclists, research suggests that reducing the posted speed limit from 35 mph to 30 mph can reduce injuries to bicyclists by 17-32% and fatalities by 21-45% (Grembek et al., 2020).

4. Federal and State Guidelines on Safe Speeds for Local Governments

There is growing consensus among academic researchers and across various levels of government around using the Safe System and Safe Speeds approaches to set speed limits to address traffic safety. Prior to recent changes, jurisdictions in California had been constrained by state law and policy guidelines that required the use of the 85th percentile speed as the first step in setting speed limits in many instances. While the 85th percentile speed often continues to be a starting point for speed limit setting in California, recent state legislation expands upon the existing methodology in speed limit setting by giving jurisdictions more factors to consider in addition to the 85th percentile when setting speed limits.

Local vs. State Control of Roadways

An important component of the speed limit setting process is determining which entity is responsible for the road under consideration. Roads are generally classified as locally-controlled roads or state-controlled roads. State-controlled roads, or roads on the “State Highway System” are generally highways and other arterials that have relatively higher speed limits and carry traffic between multiple municipalities. In California, state-controlled roads are managed by the California Department of Transportation (Caltrans). In contrast, locally-controlled roads generally tend to carry relatively smaller volumes of traffic when compared to state highways and tend to begin and end within the boundaries of the governing jurisdiction. While state-controlled roads generally tend to carry larger volumes of traffic, there is a wide range of road types that qualify as “locally-controlled roads,” and these can range from larger arterials, to mid-sized collectors, to the smallest volume roads which are called “local roads.” Every California road classification can be viewed on the Caltrans California Road System Map

Responsibilities for maintenance and operations on certain segments of state-controlled roads may, in some cases, be relinquished to local jurisdictions. This is more commonly seen on state-controlled roads that cross through large cities such as Los Angeles or Sacramento. Occasionally, state-controlled roads may be referred to as “county roads” if a county is responsible for managing the road or a section of it. While certain segments of state-controlled roads may be colloquially referred to as “local roads” by residents if the state has relinquished responsibility over that section to a local jurisdiction, we refer to these sections of state-owned roads under local governance as “locally-maintained state roads” rather than “local roads” to enhance clarity.

Speed Limit Setting in California & Recent Legislation

Jurisdictions in California are generally constrained in their ability to set speed limits by state laws and regulations, including provisions in the California Vehicle Code (CVC), the California Manual on Uniform Traffic Control Devices (CA MUTCD) (Caltrans, 2026), and guidance from the California Manual for Setting Speed Limits (CMSSL) (Caltrans, 2026).

The requirements and guidance across the CVC, CA MUTCD and CMSSL include context-specific prima facie speed limits, Engineering and Traffic Study (E&TS) procedures, allowable deviations from the 85th percentile speed, signage requirements, and radar enforceability. An important exception to these requirements relate to local roads (as defined under the California Road System maps), which do not require an E&TS to designate a speed limit and are assigned speed limits deemed appropriate by local transportation engineers.

In addition to local roads, which do not require an E&TS, some corridors qualify for prima facie speed limits, which are speed limits that are designated by law for certain types of roadways (e.g., alleys, school zones, senior zones). Many of these prima facie speed limits apply even in the absence of actual speed limit signage. Some roads have a prima facie speed limit of 20 or 25 mph (such as those in senior zones, school zones and business activity districts), and are therefore excluded from certain speed trap provisions (CVC § 40802).

Outside of local roads and prima facie zones, opportunities to comprehensively lower speed limits without an 85th percentile speed-based E&TS are limited. Importantly, roads that carry higher volumes, such as arterial or collector roads, still do require speed limits based on the 85th percentile speed through an E&TS when no prima facie speed is designated. These high-volume roadways in urban settings are more likely to be on a jurisdiction’s High Injury Network due to higher serious injury and fatal crash incidence. Jurisdictions may find safe speed limit setting and protecting vulnerable users especially challenging on these roadways when prevailing driver speeds are higher than is safe for all roadway users.

Recent state legislation (AB 43, AB 1938, AB 1014, AB 382, and AB 321) has given jurisdictions more flexibility in setting speed limits on both state-controlled and locally-controlled roads in three major ways:

The creation of new prima facie zone legislation allowing lower speed limit designations in business activity districts and school zones.
Granting jurisdictions the ability to further lower speed limits in certain safety-related settings relative to 85th percentile speeds measured during the E&TS process.
Granting jurisdictions the ability to consider retaining current speed limits or restoring the immediately prior speed limit on a corridor, especially if it was raised in the past 10 years, even if the current E&TS yields a higher 85th percentile speed.

Federal Policy Direction

In 2022, the US Department of Transportation’s National Roadway Safety Strategy (USDOT 2022) adopted the Safe System Approach as a guiding paradigm to address traffic fatalities and serious injuries nationwide. One of the action areas led by the Federal Highway Administration (FHWA) is to “promote safer speeds for all users through context-appropriate speed limits, road designs, and other practices” (USDOT 2022).

The 11th Edition of the federal Manual on Uniform Traffic Control Devices (MUTCD) was issued in December 2023. It expanded upon the approach for setting speed limits, emphasizing the need to consider the roadway context, especially in urban areas where the 85th percentile speed is higher than the desired speed limit due to the presence of vulnerable road users. This new edition downgraded many of the guidelines (“should” statements in the prior version) to “support” statements (i.e., information to consider). The federal MUTCD does not have force or authority in California; instead, California revised the CA MUTCD in 2026 to be consistent with the federal version.

The 2026 CA MUTCD continues to recommend using “the 85th percentile rule” as a starting place to set speed limits, though it was recently updated to reflect recent state legislation that provides jurisdictions more flexibility to deviate from the 85th percentile speed, if deemed appropriate. This is critical because the National Transportation Safety Board (NTSB) found that relying on the 85th percentile speed to change speed limits in high-speed zones results in “higher operating speeds and new, higher 85th percentiles in the speed zones, and an increase in operating speeds outside the speed zones” (Grembek et al., 2020, p. 54; Caltrans, 2026; NTSB, 2017). The NTSB recommended incorporating the Safe System Approach for urban roads to strengthen protection for vulnerable users (NTSB, 2017).

Caltrans Director’s Policy and the 2020-24 Strategic Highway Safety Plan

In 2022, Caltrans issued Director’s Policy 36, which adopts the Safe System Approach as the framework for achieving the vision to eliminate fatalities and serious injuries on California’s roadways by 2050 (Caltrans, 2022). This policy directs all Caltrans divisions to align their practices with and promote the implementation of the Safe System Approach (Caltrans, 2022).

Caltrans’ 2020-24 Strategic Highway Safety Plan incorporates the Safe System Approach as a guiding principle to addressing various High Priority Challenge Areas, including Speed Management/Aggressive Driving and Active Transportation Safety (Caltrans, 2023). Specifically, the Implementation Plan for Speed Management and Aggressive Driving called for implementing a new roadway-based, context-sensitive approach to establish speed limits in California that prioritizes the safety of all road users (Caltrans, 2023).

Zero Traffic Fatalities Task Force

In 2019, the California State Transportation Agency (CalSTA) established the Zero Traffic Fatalities Task Force (ZTFTF) under AB 2363 to develop policy recommendations for reducing statewide traffic fatalities to zero (California State Transportation Agency, 2023). The task force assessed procedures in place at that time for setting speed limits in California, which relied exclusively on “the 85th percentile rule” and recommended greater flexibility for local jurisdictions to set speed limits.

One of the ZTFTF’s long-term recommendations is to develop a new roadway-based, context-sensitive approach to establishing speed limits that prioritizes the safety of all road users. The report also recommended various changes to speed limit setting, such as increasing the reduction allowance for posted speed limits to allow for greater deviations from the 85th percentile speed. Some of these recommendations were incorporated into legislation that prioritized multimodal safety and provided jurisdictions with greater flexibility in setting speed limits and are now reflected in the 2026 revision of the CA MUTCD.

5. Integrating Safe Speeds and Safe Road Design

Integrating safe speed limits, engineering improvements, and effective and equitable enforcement can be a productive suite of strategies for jurisdictions seeking to comprehensively address safety challenges and improve safety outcomes. Whereas a safe speed limit allows engineers to appropriately design the roadway for such speed, engineering improvements in turn prompt drivers to drive at a safe speed. This integrated approach is an important part of the Safe System Approach, which recognizes that one single element may not be sufficient to improve safety, and that a comprehensive approach is necessary to create redundancies that accommodate human mistakes (FHWA 2022).

California adopted the Safe System Approach. Their Safe System wheel includes Safer Land use in addition to the USDOT element to support the impact of land use decisions on safety outcomes. The updated Safe System wheel is shown below, as referenced from the California Strategic Highway Safety Plan (SHSP)

Figure 6. The Safe System Approach adapted for California

Infographic detailing the 6 main components of the Safe System Approach including safe road users, safe vehicles, safe speeds, safe roads, safer land use, and post-crash care.

Source: California Strategic Highway Safety Plan (Caltrans, 2026)

While reducing the posted speed limit is a proven mechanism to reduce actual speeds (Grembek et al., 2020, p. 21), most studies suggest that effective speed management also relies on other factors such as roadway geometry, traffic control devices, traffic calming devices (vertical and horizontal deflection, such as speed tables and chicanes), signage, enforcement, and land use (Grembek et al., 2020, p. 21).

Local jurisdictions have the flexibility to implement safe speeds through engineering approaches on roads under their control. For example, providing appropriately-sized lane widths can be an effective low-cost strategy for reducing speeds that is supported by research and design standards (Hamidi, S. et al., 2023). From a research perspective, narrower lanes are associated with lower speeds and do not increase crash rates on urban arterials (NACTO 2013). As for design standards, AASHTO’s “Green Book'' considers 10ft travel lanes generally acceptable for urban local, collector, and arterial roads (AASHTO 2018). This design strategy is also supported by NACTO (2013)and ITE (n.d.). Another tool jurisdictions have that pairs well with reductions in posted speed limits is signage — specifically, increasing the density of speed limit signage accompanied by the use of automatic speed feedback signs to display “YOUR SPEED” for drivers.

An international example of this integrated approach comes from the Netherlands, which recommends posting Safe Speed Limitsthat are also Credible Speed Limits (SWOV Institute for Road Safety Research, n.d.). A Credible Speed Limit is a limit that the majority of drivers consider a logical speed for that specific type of road in that specific road environment (SWOV Institute for Road Safety Research, n.d.). If the Credible Speed Limit is higher than the Safe Speed Limit, engineers would work to adjust the road environment to meet the lower speed limit instead of increasing it, if at all possible.

Many jurisdictions across the US are already implementing this integrated approach. The City of Seattle reduced the number of crashes on Greenwood Avenue by 35% by reducing the speed limit from 30 mph to 25 mph, posting additional speed limit signs, and installing new signal heads, sidewalks, and intersection bike markings. NCHRP 17-76 concluded that tying speed limits to the 50th percentile speed may be a more sensible approach than using the 85th percentile speed for select roadway types (Fitzpatrick et al., 2021).

Disclaimer

This Toolkit does not constitute a standard, specification, or regulation. This Toolkit is not intended to replace the existing Caltrans mandatory or advisory standards and is not intended to be a substitute for engineering knowledge, experience or judgment. This Toolkit promotes better access to helpful information and concepts from various agencies and organizations. UC Berkeley and Caltrans acknowledge the existence of other practices and provide this Toolkit for reference and direction for those responsible for making professional engineering or other design decisions.

Please be aware that neither UC Berkeley nor Caltrans is the author of the external sources contained in this Toolkit and are therefore not responsible for errors or omissions that may be contained within those external sources. The references contained in this Toolkit are provided for the user’s convenience. The content or information in such references is the sole responsibility of their respective authors and/or owners.

The toolkit was originally developed with funding from the California Department of Transportation (Caltrans). The latest update of this toolkit, dated September 2026, is funded by the California Office of Traffic Safety (OTS), through the National Highway Traffic Safety Administration (NHTSA).