| Class/Subclass: | Freeway | Arterials | Collectors | Local | |
| Item | Principal | Minor | |||
| Anticipated Speed or Speed limit: | 55 to 70 mph | 45 to 55 mph | 30 to 45 mph | 30 to 40 mph | = 30mph |
| Purpose: | High mobility, access limited to ramps | High mobility, limited access | Balance between mobility and access | Connects local roads to arterial | Permits access to abutting land |
| Cross Section: | Multilane divided | Multilane divided or undivided | Multilane divided or undivided; one-way; two-lane | Two- or three-lane with curb and gutter | Two-lane with curb and gutter |
| Signals per mile: | N/A | 1 to 2 | 4 to 6 | > 6 | None |
| Access Density (typical spacing per direction): | none (ramps) | Low (600 ft) | Moderate (400 ft) | High (200 ft) | High (100 ft) |
| Parking: | No | No | Some | Usually | Yes |
| Roadside Development: | None | Low density | Moderate density | High density | Residential/parks/schools |
| Median: | Yes, restrictive | Yes, restrictive | Usually | Some, unrestrictive | No |
| Sidewalks: | No | Preferred | Yes | Yes | Preferred |
| Anticipated traffic: | High volumes | High volumes | Moderate to high volumes | Low to moderate volumes | Low volume with access by garbage and fire trucks |
| Bike Lanes: | No | No | Maybe (or wide curb lane) | Maybe (or wide curb lane) | No |
pp87-88
Field Studies
Free-flow speed data were collected at 78 sites in S/U and rural areas in seven cities located in six states. For each site, roadway and roadside characteristics were also collected, such as number of access points within the study section, roadside development type, and lane width.
Initial graphical evaluation provided a visual appreciation of potential relationships between a roadway or roadway variable and operating speed. Findings from the evaluation included the following:
Posted speed limit: This has the strongest relationship to 85th percentile speed. As posted speed increases the 85th percentile speed increases.
Functional class: Local roads had the lowest speeds collected, while arterials had the highest.
Access density (the number of access points, such as driveways and intersections, per mile): It showed a strong relationship with 85th percentile speed, with higher speeds being associated with lower access densities.
Pedestrian activity: Lower speeds occur as pedestrian activity increases.
Centerline or edgeline markings: The absence of either is associated with lower speeds.
On-street parking: When permitted, speeds are lower.
Median: When present, speeds are slightly lower than when a raised, depressed, or TWLTL is present, with a few exceptions.
Distance between features that have influence on a drivers speed, such as a signal or sharp horizontal curve: As the distance increases, speeds increase to a point and then plateau.
Shoulder width: Roadways with shoulders that have widths of 6 ft (1.8 m) or more have speeds above 50 mph (80.5 km/h), with one exception. Roadways with shoulders between 0 and 4 ft (0 and 1.2 m) also had speeds up to 50 mph (80.5 km/h), but most speeds observed were lower than on the roadways with wider shoulders. Roadways with curb and gutter had speeds across the entire range seen on roadways with shoulders (25 to almost 60 mph [40.2 to almost 96.6 km/h]). There is no evidence that the presence of curb and gutter results in lower speeds for a facility.
Signal density: Higher signal density is associated with lower speed.
Pavement width: Fewer lower speeds are associated with wider pavement.
Median width: Fewer lower speeds are associated with wider medians.
Lane width: No relationship was apparent between lane width and speed.
Type of residential development: Speeds on streets with single-family residential development tended to have lower speeds; however, some sites with residential development had speeds near 50 mph (80.5 km/h). A sizeable range of speeds occurred within each development type.
The statistical evaluation began with attempting to predict operating speed using the collected roadway and roadside variables.
The site variation in operating speeds is highly correlated with the variation in posted speed limits.
Access density had a t-statistic greater than 1, which corresponded to an approximate 20 percent alpha level.
No other roadway variable was statistically significant at the 20 percent alpha level or higher. Despite their low t-values, however, several variables do show some sign of influence on the 85th percentile freeflow operating speed, including median type, parking along the street, and pedestrian activity level. One encouraging aspect of this analysis is that regardless of the low t-values, most of the estimated regression coefficient values do have expected algebraic signs. This suggests that the influences of these variables on the 85th percentile free-flow operating speed are likely to be there, and the reason for not being able to estimate them to a good statistical accuracy is most likely due to the limited number of sites available for analysis.
A cluster analysis was performed to determine if the project team could gain additional insight on the perceived influences of a roadway attribute on operating speed. The analysis resulted in a seven-cluster model. The following were the noteworthy features found within the analysis: pedestrian activity, parking, centerline, median treatment, roadside development, area type, and signal density.
A strong limitation with the speed relationships is the amount of variability in operating speed that exists for a given design speed, for a given posted speed, or for a given set of roadway characteristics.