Downstream Effects of Speed Photo—Radar Enforcement and Other Speed Reduction Treatments on Work Zones

The effects of automated speed photo—radar enforcement (SPE) and traditional speed reduction treatments (speed feedback trailer, presence of police vehicles with emergency lights on and off, and combinations of the speed feedback trailer and police presence) on speed were studied at a location 1.5 mi downstream of the actual treatment (spatial effects). Three data sets from two Interstate highway work zones were used. Field data consistently showed significant spatial (downstream) effects for SPE. The combination of speed feedback trailer and police vehicle with emergency lights off had downstream effects in some cases but to a lesser degree than SPE. Other treatments showed no significant downstream effects. For free-flowing traffic, SPE reduced the average downstream speed by 2 to 3.8 mph for cars and by 0.8 to 5.3 mph for trucks. Also, SPE reduced speeding cars by 7.1% to 23.4% (except for cars in median in Data Set 1), and speeding trucks by 4.2% to 48.3% (except for trucks in shoulder in Data Set 3). For the general traffic stream, SPE reduced the average downstream speed by 1.1 to 2.9 mph for cars and by 0.9 to 3.3 mph for trucks. When SPE was used, the percentage of speeding cars and trucks in the general traffic stream was reduced by 2.9% to 28.6%, and by 7.5% to 36.1%, respectively. SPE also reduced the percentage of cars in the general traffic stream exceeding the speed limit by more than 10 mph in virtually all cases, and eliminated such trucks in all but one case.