Estimating Free-flow Speed from Posted Speed Limit Signs

In 2010 Highway Capacity Manual, one preferably determines free-flow speed (FFS) by deriving it from a speed study involving the existing facility or on a comparable facility if the facility is in the planning stage. Many have used a ‘rule of thumb’ by adding 5 mi/h (10 km/h) above the posted limit to obtain FFS without justification. Two team members using a radar gun and manual tally sheets collected 1668 speed observations at ten sites during several weeks. Each site had a unique posted speed limit sign ranging from 20 mi/h (30 km/h) to 75 mi/h (120 km/h). Five sites were on urban streets. Three sites were on multilane highways, and two on freeways. Goodness-of-fit test results revealed that a Gaussian distribution generally fit the speed distributions at each site at a 5% level of significance. The best-fit model had a correlation coefficient of +0.99. The posted speed limit variable was significant at 5% level of significance. Examining data by highway type revealed that average free-flow speeds are strongly associated with posted speed limits with correlation coefficients of +0.99, +1.00, and +1.00 for urban streets, multilane highways, and freeways, respectively.