Geometric Design

Geometric Design

Geometric P SU/ WB-67
P SU WB-67

The geometric design of ECFs depends on the design speed, roadway classification, and the design vehicle. The type of ECF, available space, and traffic volume may impact these design criteria. A design speed of 25 mph is ideal, but there are cases when it can be lowered to 15 mph. Roadway classifications include freeway, arterial, collector, and local. In most cases, roadways approaching ECFs are collector roadways. The design vehicle is the largest vehicle likely to use the facility with considerable frequency. For the purpose of ECFs, a design vehicle WB-50 to WB-67 should be used in areas that accommodate trucks. At ECFs where trucks are not permitted, a single unit truck (SU) or a bus (school, transit, or other) may be the appropriate design vehicle however, a pre-ID check area turnaround capable of rejecting a truck should be provided.

Travel Way and Lane Width

As a general rule, when free flow through an ECF is desired, travel lanes should be at least 12 feet wide. Narrower lanes approaching and departing the ID check area will restrict traffic flow and contribute to congestion. At the ID checkpoint itself, a lane width of 10 feet will not impede traffic flow, as congestion is inherent with stopping or slowing.

Other considerations regarding 10-foot lanes include:

  • Narrow lanes are highly restrictive to large vehicles, including some emergency vehicles.
  • Narrow lanes can impact traffic flow. When the lane width is less than 12 feet, motorists drive very cautiously and also tend to increase the spacing between vehicles.

Snow removal requirements including the necessary width between ID check islands for snow removal equipment should be considered. If moderate-to-heavy bicycle traffic is expected, a 5-foot bike lane between the travel lane and gutter is recommended.

Travel Way and Lane Width

Transition Tapers

Properly constructed tapers enhance safety and efficiency. They allow drivers to recognize a change in conditions and to react accordingly. Tapers should be used whenever travel lanes are redirected, added, or dropped.

Type Usage Criteria
Lane redirected

Lane Redirected

At an ECF to accommodate ID check islands L = 100 feet minimum
40 mph or less use L = WS2/60
Greater than 40 mph use L = WS

Lane dropped

Lane Redirected

After an ECF when reduction in lanes is needed
L = 100 feet minimum


40 mph or less use L = WS2/60
Greater than 40 mph use L = WS
Lane added

Lane Redirected

Before an ECF when more processing lanes are needed Use L = WS/3 or 10:1 ratio taper, whichever is greater
Shoulder-to-curb transition

Lane Redirected

Approaching an ECF when an uncurbed section of roadway is transitioned to a curbed section in order to control vehicle movements 10:1 minimum or Lmin = 0.33L, where L=WS2/60 (40 mph or less) or L=WS (>40 mph), whichever is greater

 

Previous SectionNext Section