How are speed limits set on roadways in California?

There are many factors that play a roll in drivers' perception of the safe speed and the operation of a vehicle.  All of these factors should be looked at and taken into account as a whole when considering the establishment of speed limits on roadways. (California Manual for Setting Speed Limits)

Set Speed Limits Based on Design and Physical Characteristics of the Roadway

The physical characteristics on a roadway influence the driver's perception of the safe speed of the vehicle. These characteristics include:

• Roadway geometrics (width curves), shoulder condition (paved or unpaved and width), grade (uphill or downhill), alignment, and sight distance (visibility of approaching traffic from side street, driveways).
• Roadside development (commercial or residential), zoning (density), and environment.
• Parking practices ( parked vehicles) and pedestrian activity ( high ped activity or low).
• Driveway density (egress or ingress traffic).
• Intersections (size of intersections and traffic volume)
• Rural, residential or developed areas.

The presence of horizontal and vertical curves may influence operating speeds for a short roadway segment.  In such situations, changing the posted speed limit may not be the best option. Usually an advisory speed limit sign may be posted to inform drivers of a recommended operating speed. A recommended operating speed is not regulatory speed it is just intended to warn the drivers to reduce speed to safely negotiate the curve. To determine a recommended advisory speed for a horizontal curve a ball bank indicator or digital inclinometer (see CA MUTCD, Chapter 2C) should be used. If the measurement of the proper speed indicates that an advisory speed sign is needed such signs may be posted in advance of a curve. Please refer to the Highway Design Manual (HDM) or American Association of State Highway and Transportation Officials Policy on Geometric Design of Highways and Streets for recommended geometric criteria for specific design speeds.

Narrow traffic lane width, condition of pavement, presence and width of shoulders, distance between and frequency of intersections, and roadside development all impact the speed on a roadway but are not so easily measured. The appropriate speeds for these factors should be based on engineering judgment and measured by spot speed surveys of vehicles using the roadway.

Vehicle and Speed Limit

Technology improves the operating capabilities of vehicles. These improvements on vehicle performance obviously effect the drivers' confort level at which they drive on a roadway. For example, vehicle disk brake technology has had a huge impact on vehicle safe stopping distance which has effected the drivers perception of safe speed for stopping at shorter distance thereby effecting the operating speed.  The suspention syatem and body roll angle of different vehicles also affects the operating speed on curves.  The general practice for traffic engineers is to determine the best speeds that representing the majority of vehicles using the roadway. 
 
Driver and Speed Limit

There is a wide range of operating capabilities for inexperienced to the professional driver. It would not be prudent to set speed limits for drivers at either of the two extremes. Therefore, it makes more sense to set speed limit that is aimed at the ability and performance of all drivers. Calculations to determine critical approach speeds toward traffic control signals, crosswalks, and locations with limited sight distance use an average driver’s perception-reaction time.

Traffic

The presence of other vehicles on the highway, including those that may be entering, crossing, making turns, or on-street parking, can affect speeds. Posted speed limits should be based on off-peak hour traffic under free-flow conditions on an average weekday for the purpose of the procedures outlined herein. During congested times, drivers will adjust their speeds in accordance with the basic speed law, and are operating their vehicles at slower speeds, compared to free-flow conditions. Lowering the posted speed limit during congested periods places unnecessary limitations on vehicles during uncongested times. According to Missouri Department of Transportation study RI08-025, variable speed limits based on time of day are confusing to the driver and are difficult to enforce (See Appendix Section D.1).

Weather and Visibility

Posted speed limits are applicable under good weather conditions and dry pavement. CVC Sections 22350 and 22363 allow temporary posting of lower speed limits for snow or ice conditions on a highway. Except in cases where the posted speed limit is the statewide maximum legal limit, speeds are posted on the basis of daylight speed values determined under good weather conditions.  On highways that fall under their respective jurisdiction, Caltrans and local authorities may determine and declare a prima facie speed limit of 40, 35, 30, or 25 miles per hour, whichever is found most appropriate and is reasonable and safe based on the prevailing snow or ice conditions upon such highway or any portion thereof.

Other Roadway Users

The frequency of pedestrians is likewise an important factor, particularly at intersections with limited sight distance and in areas with crosswalks with no intersection traffic controls. The presence of bicycle traffic and the width of available shoulder or designated bicycle lanes is also an important factor. In areas with high bicycle and pedestrian use, drivers should adjust their speeds to anticipate both expected and unexpected movements into moving traffic.  These speeds should be reflected in the measured speeds during a spot speed survey. 

Vehicle Operating Speeds

Drivers adjust their vehicle speed to the readily apparent conditions present on any particular roadway.  The spot speed survey measures prevailing speeds and is a way to determine how drivers are reacting to roadway conditions readily apparent to them.