Recent research has found that the free-flow speed on a freeway depends on the traffic and roadway conditions present on a given facility. These conditions are described in the following sections.
Lane Width and Lateral Clearance
When lane widths are less than 12 feet, drivers are forced to travel closer to one another laterally than they would normally desire. The effect of restricted lateral clearance is similar. When objects are located too close to the edge of the median and roadside lanes, drivers in these lanes will shy away from them, positioning themselves further from the lane edge. This restricted lateral clearance has the same effect as narrow lanes: it forces drivers closer together laterally. Drivers have been found to compensate by reducing their speed. The closeness of objects has been found to have a greater effect on drivers in the right shoulder lane than on those in the median lane.
Drivers in the median lane appear to be unaffected by lateral clearance when minimum clearance is 2 feet, whereas drivers in the right shoulder lane are affected when lateral clearance is less than 6 feet...
Number of Lanes
The number of lanes on a freeway section influences free-flow speed. As the number of lanes increases, so does the opportunity for drivers to position themselves to avoid slow-moving traffic. In typical freeway driving, traffic tends to be distributed across lanes according to speed. Traffic in the median lane or lanes typically moves faster than in the lane adjacent to the right shoulder...