The aim of the study was to compare the decision times for left–right decisions for a dual-coded advisory turn indicator and a typical spatial-only turn indicator in a GPS navigational map display.

Track-up maps are useful for turn decision making but do not facilitate configural knowledge acquisition of an area. North-up maps present a stable orientation for this type of learning, but typical implementations of north-up map displays lead to misaligned and confusing turn information. We compared a typical spatial-only indicator with a dual-coded spatial-plus-verbal indicator, systematically manipulating vehicle heading and measuring reaction time. The new display, the Dual-Coded Advisory Turn Indicator for Maps (DATIM), was based on an assumption of the advantages of concurrent verbal and spatial processing of advisory turn indicators in map displays.

The experimental design was a 2 x 2 x 24 mixed design with indicator type as a between-subjects factor and turn direction (left, right) and 24 heading angles (15° intervals) as repeated-measures factors. Participants made turn decisions while viewing static displays of intersections at variably rotated headings.

Reaction time for the DATIM display was consistently faster than the typical spatial-only indicator at all heading angles but especially at heading angles beyond ±45° (520-ms difference at 180°).

The DATIM display produced faster turn decisions at all heading angles.

DATIM displays could allow north-up maps to be used for turn-by-turn decision making in GPS navigational systems. Drivers could have the advantages of both the stable orientation to facilitate planning and the easy turn-by-turn guidance. Limitations are discussed.