One-way traffic advantages and conflict points

Advantages of one-way traffic

• Fewer conflict points (a four-arm junction of two one-way streets has far fewer than the 32 of two-way streets), so fewer and less severe crashes.
• Higher capacity (typically 20-50 percent more) as no opposing flow, no right-turn crossing and less need for signal phases.
• Easier signal coordination and progressive flow; fewer signal phases.
• Parking and loading can be allowed on both sides without blocking opposing flow.
• Higher speed and less delay; less glare at night; fewer head-on and sideswipe crashes.
• Pedestrians only look in one direction when crossing.
• No central median needed; easier lane use.

Limitations: longer travel distance for some trips, confusion for strangers, and need for a parallel return street.

Conflict points at a four-arm intersection (both roads two-way)

              N
          |  D | M  |
          |    |    |
  ------- X--X-X--X --------
   M D    X  X X  X     D M
  ------- X--X-X--X --------
   D M    X  X X  X     M D
  ------- X--X-X--X --------
          |    |    |
          |  M | D  |
              S
 X = crossing, M = merging, D = diverging
 (4 diverging + 4 merging near arm ends; 
  crossing points in the central area)

Each of the four approaches has through, left-turn and right-turn movements (12 movements in all).

Classification of conflicts

1. Diverging: one stream splits into two (a vehicle leaves the approach stream to turn). Least severe; 2 per approach = 8.
2. Merging: two streams join into one at an exit. Moderately severe; 2 per exit = 8.
3. Crossing: two streams cut across each other's path (right turners across opposing through traffic, and cross-road through/turning movements). Most severe and most numerous = 16.

Hence crossing conflicts (half of all) are the main target for control by signals, islands, rotary or grade separation.