1	“Social Costs” Theoretical Comparison of Signalized Intersections to Roundabouts with similar traffic flows (single-lane, 4-way, same vpd) By Prof.R.N.White Math/CSC Dept, Eastern Ct State Univ. (ECSU) December’03
2	Premise Lets assume 4-way single-lane intersections with similar traffic flows (vpd) Pedestrians are important but not fully considered It would be very useful to compare the social costs of 1000 crashes at Signalized Intersections to 1000 crashes at Roundabouts (although it is documented that Roundabouts reduce total #’s of crashes – see slide#8) Some actual data supports the notion that Social Costs are proportional (Y) to )V²
3	Conflict Points - Signalized Merging crashes ~33% Low angle (0^o- 45^o) Low differential speed (0-10mph) Crossing (T-bone) ~33% Rt angle (90^o) ~30mph Crossing (Head-on) ~33% High angle (90^o – 180^o) High total speed (30-60mph) “Diverging” crashes for 2 lane
4	Conflict Points - Roundabout Merging crashes 100% Low angle (0^o- 45^o) Low differential speed (0 -10mph) “Diverging” crashes for 2 lane
5	Crash Energy The entire Kinetic Energy of a crash must be dissipated! Heat & Noise Bending, crushing, & ripping materials Property damage KE = ½ mv², thus “damage” Y v²(crash speed squared) “Social cost” Y damage
6	Damage Y Social cost per 1000 crashes in each Signalized Intersection (3 parts) 33% @ 0-10mph => 50units 33% @ 30mph => 900units 33% @ 30-60 => 900-3600 (average = 2250units) Sum = 3200; Average = 1067units For 1000 crashes, damage Y 1,067,000 units Roundabouts (1 part) All crashes @ 0-10mph Thus v²= 0 – 100, or average of 50units For 1000 crashes, damage Y 50,000 units
7	Social Cost Comparison per 1000 crashes Since the total v² for signalized intersections is 1,067,000 units For Roundabouts with similar traffic flow, v² is 50,000 units Ratio = (1,067,000 units)/(50,000 units) = 21 The “Social Costs” for signalized intersections is ~21 times higher than for Roundabouts (assuming both 4-way single lane, same vpd)
8	Crash Comparison Annually
9	Social Cost Comparison (4-way, single lane) per Intersection-Year (based on Slide #8) Using the data reported on the last slide, we would expect … Total Crash Ratio = Signal/RDB = 2 Social Cost / Year = (Crash Ratio) * (Social Cost Ratio) = (2) * (21) = 42 PI Crash Ratio = Signal/RDB = 4 Fatality Ratio = ? PDO Crash Ratio = Signal/RDB = 1.5
10	Pedestrian considerations
11	Conclusion (1) - Real Cost of a “Reconfiguration” If you assume a Reconfiguration might be redone every 20 years for a “difficult” intersection, the “Real Cost” to society p.a. is the sum of: Construction/20 Annual maintenance costs Annual “Social Costs” (besides crash costs, environmental factors may be evaluated & added to this) Unnecessary pain & suffering (no $ value possible!)
12	Conclusion (2) Do you want to be responsible for unnecessary PD, PI’s, & Fatalities?