
Measuring Roadway Curve Radius
Using the Compass Method by John C. Glennon and James R. Loumiet January 2003 A very common method used by accident investigators is to calculate roadway curve radii from chordoffset measurements taken in the field. The ChordOffset Method is taught in most of the major accident reconstruction schools and is described in the Northwestern Traffic Institute's 1990 book, Traffic Accident Reconstruction, by Lynn B. Fricke. The ChordOffset Method usually uses a 100foot tape held on either end at the precise edge of the roadway, while a carpenter's rule is used at the middle of the tape to measure the distance between the edge of the tape and the edge of the roadway. These two measurements, the 100foot chord length and the measured middle offset, are then used in the following equation to compute the radius: R = L^{2} /8m + m/2
Where R = the roadway curve radius, feet
L = the chord length, feet m = the measured middle offset, feet
R = 57.3L/D_{C}
Where R = the roadway curve radius, in feet
L = the roadway curve length in feet, and D_{C} = the change in roadway direction, in degrees. From a practical standpoint, a measuring compass and a measuring wheel are easier and safer to handle in the field than a measuring tape. The Compass Method is also a one person task, as opposed to the two or three people needed to accurately make chordoffset measurements. With the Compass Method, the roadway curve can be safely measured from the shoulder using the measuring compass and measuring wheel. After the radius, R, is calculated, the investigator can then add or subtract the appropriate distance to the calculated radius to determine the radius of any arc on the roadway. For example, if the radius of a curve measured at the inside edgeline is 580 feet, and the normal distance between that edgeline and the centerline is 12 feet, then the radius of the centerline is 592 feet. During this entire operation, the investigator never has to enter the roadway, or stoop on the pavement to make measurements. In summary, the Compass Method of measuring radii of roadway curves is a safe, accurate, simple, quick, and inexpensive method for determining the radius of roadway curve, and as such is an attractive technique for accident investigators. About the Author
Dr. John C. Glennon is a traffic engineer with over 45 years experience. He has over 120 publications. He is the author of the book "Roadway Safety and Tort Liability" and is frequently called to testify both about roadway defects and as a crash reconstructionist. RESUME OF JOHN C. GLENNON john c. glennon, Book , books,pavement edge drop expert, pavement edge drop off expert, guardrail expert, work zone safety expert, construction zone safety expert, roadway hydroplning expert, traffic engineering expert, traffic sign expert, traffic signal expert, pavement marking expert, highway safety expert 
