THE EFFECT OF LETTER WIDTH AND SPACING ON NIGHT LEGIBILITY OF HIGHWAY SIGNS

THE OBJECT IS TO DETERMINE THE EFFECT THAT SPACING BETWEEN LETTERS OF WORDS, USED IN HIGHWAY SIGNS, HAD ON THEIR NIGHT- TIME LEGIBILITY. MORE THAN 2,500 OBSERVATIONS WERE MADE BY 36 OBSERVERS WHILE DRIVING AN AUTOMOBILE AT 30-MPH. WHITE REFLECTORIZED LETTERS, 10 IN. HIGH, WERE DISPLAYED ON A BLACK NONREFLECTORIZED BACKGROUND. THREE DIFFERENT ALPHABETS WERE USED. TWO OF THESE, THE STANDARD SERIES C WITH NARROW LETTERS AND THE WIDER SERIES E, WERE CUT FROM REFLECTIVE SHEETING. THE THIRD ALPHABET, IDENTIFIED AS SERIES ED AND SIMILAR IN WIDTH TO THE SERIES E, WAS DESIGNED BY A MANUFACTURER USING 1 1/4-INCH DIMATER PLASTIC REFLECTORS TO FORM THE LETTERS. THE SPACINGS BETWEEN LETTERS WERE INCREASED AS THE LENGTHS OF THE SIX TEST WORDS WERE EXTENDED FROM NORMAL TO 20, 40, AND 60 PERCENT ABOVE NORMAL. AS INTERLETTER SPACING WAS INCREASED, THE LEGIBILITY DISTANCES ALSO INCREASED FOR ALL THREE ALPHABETS UNTIL WORD LENGTHS WERE 40 PERCENT ABOVE NORMAL. THE RESULTING GAIN IN LEGIBILITY AT THIS POINT WAS 15 PERCENT FOR SERIES C, 16 PERCENT FOR SERIES E, AND 7 PERCENT FOR SERIES ED. BEYOND THE 40-PERCENT INCREASE IN WORD LENGTH, LEGIBILITY LEVELED OFF OR DECLINED. WHEN WORD LENGTHS WERE NORMAL OR NO MORE THAN 10 PERCENT ABOVE NORMAL, TEST SIGNS WITH THE SERIES ED ALPHABET WERE FOUND TO HAVE GREATER LEGIBILITY. AT WIDER SPACINGS, THE SERIES E ALPHABET WAS SUPERIOR. AS MIGHT BE EXPECTED, THE 10-IN. SERIES E ALPHABET WAS LEGIBLE AT A GREATER DISTANCE (118 TO 142 FT) THAN THE NARROWER 10-IN. SERIES C ALPHABET AT CORRESPONDING LETTER SPACINGS. ON A PERCENTAGE BASIS, THE DIFFERENCES IN LEGIBILITY FAVORING THE SERIES E ALPHABET RANGED FROM 23 PERCENT TO 27 PERCENT. A WORD WITH LETTERS OF THE SERIES C ALPHABET IS SHORTER IN LENGTH THAN ONE WITH LETTERS OF THE SERIES E FOR A GIVEN SPACING, AND A COMPARISON OF LEGIBILITY DISTANCE PER INCH OF WORD LENGTH SHOWED THAT THE SERIES C ALPHABET WAS SOMEWHAT SUPERIOR TO THE SERIES E. ALSO STUDIED WAS THE PROBABLE EFFECT OF INCREASING THE LETTER HEIGHT OF THE NARROWER ALPHABET UNTIL THE LEGEND AREA EQUALED THAT OF THE WIDER ALPHABET. AT THE POINT OF EQUIVALENT LEGEND AREA AND SPACING, THE TWO ALPHABETS PROVED TO BE EQUALLY LEGIBLE. THE STUDY FINDINGS POINT TO THE IMPORTANCE OF SIGN PROPORTIONS AND PROVIDE AN IMPROVED MEANS FOR EFFICIENT DETERMINATION OF LEGEND DESIGN. WHERE VERTICAL DIMENSIONS RESTRICT SIGN LETTER HEIGHTS TO SOMETHING LESS THAN DESIRABLE, INCREASED SPACING BETWEEN LETTERS CAN HELP TO COMPENSATE FOR THE LOSS OF LEGIBILITY DISTANCE THAT WOULD OTHERWISE OCCUR. /AUTHOR/

  • Supplemental Notes:
    • Vol 35, pp 600-617, 19 FIG, 3 TAB, 8 PHOT, 3 REF
  • Authors:
    • Solomon, D
  • Publication Date: 1956

Media Info

  • Monograph Title: Proceedings of the Thirty-Fifth Annual Meeting of the Highway Research Board, Washington, D.C., January 17-20, 1956
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00226015
  • Record Type: Publication
  • Files: TRIS, TRB
  • Created Date: Aug 15 1971 12:00AM